# Some bitrate results: OTA vs Cable for HD locals



## moyekj (Jan 24, 2006)

I've been meaning to do this for a while now and finally got some time this pm. Since I have Tivo S3 I can record both from Cox HD locals and from OTA broadcast of the same. So to test if Cox does much bit-rate shaping/degradation of the HD locals I recorded 30 min segments at the same time, 1 tuner recording Cox, other tuner recording OTA and then recorded the total file size of each (Tivo reports file sizes of each recording). Here are some results:

```
Cox, Orange County (Mission Viejo) headend
STATION			OTA (GB)	CABLE (GB)	%DIFF
CBS (1080i)		4.00		3.87		-3.2
NBC (1080i)		3.96		3.84		-3.0
ABC (720p)		3.47		3.52		1.4
FOX (720p)		3.36		3.51		4.5
```
So the Cox versions of 1080i channels are a little smaller, but surprisingly the Cox versions of the 720p are a little larger?? In any case based on this admittedly very small sample looks like Cox OC is not degrading the HD locals by much if at all.

NOTE: There is some margin of error involved since the OTA version and Cox version are not exactly in sync (off by a few seconds), but that probably is negligible difference especially since these were 30 min samples.

NOTE: Other factor is the 1080i OTA versions have complete PSIP information while the cable versions do not. The 720p cable channels do have full PSIP. Not sure how large PSIP information is compared to total file size but thought I should point that out.


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## HDTiVo (Nov 27, 2002)

Obviously your OTA signals come from local broadcast affiliates. Where does your cable co get its feed from?


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## Joybob (Oct 2, 2007)

moyekj said:


> I've been meaning to do this for a while now and finally got some time this pm. Since I have Tivo S3 I can record both from Cox HD locals and from OTA broadcast of the same. So to test if Cox does much bit-rate shaping/degradation of the HD locals I recorded 30 min segments at the same time, 1 tuner recording Cox, other tuner recording OTA and then recorded the total file size of each (Tivo reports file sizes of each recording). Here are some results:
> 
> ```
> Cox, Orange County (Mission Viejo) headend
> ...


I guess we finally know what you do with your free time...


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## moyekj (Jan 24, 2006)

HDTiVo said:


> Obviously your OTA signals come from local broadcast affiliates. Where does your cable co get its feed from?


 The cable feeds are from the same affiliates (Los Angeles), but I think I was told once that the cable company gets the feeds via fiber optic link, not OTA. One would assume the feeds should be very similar if not identical, but I don't know for sure.


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## moyekj (Jan 24, 2006)

Joybob said:


> I guess we finally know what you do with your free time...


 The reason I post this is I read quite a lot of claims in AVS forums and here that various cable companies degrade HD broadcasts compared to OTA, but I've never noticed much difference, hence I set out to study it on my own and this file size technique is pretty solid evidence. It really doesn't take much time or effort to do so I thought why not...

Was kind of hoping others interested in this topic and with both OTA & cable feeds available may want to post their own results as a sort of informal survey.


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## Joybob (Oct 2, 2007)

moyekj said:


> The reason I post this is I read quite a lot of claims in AVS forums and here that various cable companies degrade HD broadcasts compared to OTA, but I've never noticed much difference, hence I set out to study it on my own and this file size technique is pretty solid evidence. It really doesn't take much time or effort to do so I thought why not...
> 
> Was kind of hoping others interested in this topic and with both OTA & cable feeds available may want to post their own results as a sort of informal survey.


You could argue that their compression techniques are degrading the quality of the transmission without decreasing the bandwidth requirements?

Similar file size doesnt mean it's the same quality?


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## sfhub (Jan 6, 2007)

moyekj said:


> NOTE: Other factor is the 1080i OTA versions have complete PSIP information while the cable versions do not. The 720p cable channels do have full PSIP. Not sure how large PSIP information is compared to total file size but thought I should point that out.


Can you compare just the video/audio streams using TSReader Lite?


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## moyekj (Jan 24, 2006)

sfhub said:


> Can you compare just the video/audio streams using TSReader Lite?


 Well the problem there is that requires a PC, and since I only have 1 PC tuner I can't capture OTA & QAM at the same time like I can do with the Tivo. When TTG is enabled in November I suppose then I could transfer both streams to my PC from Tivo, but then the extracted streams will not be in Transport Stream format - they will likely be in encrypted .TiVo format as with S2 captures. There are programs to decrypt .TiVo to .mpg and then I suppose I could convert to .ts but as you can see that's quite a lot of hassle to do all that for this purpose...


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## moyekj (Jan 24, 2006)

Joybob said:


> You could argue that their compression techniques are degrading the quality of the transmission without decreasing the bandwidth requirements?
> 
> Similar file size doesnt mean it's the same quality?


 I highly doubt they are transcoding the HD feeds. Most often there is some bitrate shaping going on more than anything else, see:
http://broadcastengineering.com/infrastructure/broadcasting_dtv_digital_cable/


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## HDTiVo (Nov 27, 2002)

While I do not have access to OTA to offer comparisons, I'd like to mention that I find some variability on CBS-cable. Most prime time recordings are around 5.25-5.5GB/hr while Sunday football has been around 6-6.25/hr this year.

I've seen 4 1/3/hr to over 6/hr on ESPNHD. Bronx is Burning was generally the low end, while football the higher end.


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## aaronwt (Jan 31, 2002)

Here Ota is the same size as Comcast which is the same size as FIOS on the same recording. And visually they all look the same.


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## sfhub (Jan 6, 2007)

moyekj said:


> Well the problem there is that requires a PC, and since I only have 1 PC tuner I can't capture OTA & QAM at the same time like I can do with the Tivo.


Have any friends with PC tuners in your area?


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## moyekj (Jan 24, 2006)

sfhub said:


> Have any friends with PC tuners in your area?


 Not worth all the hassle. I think comparing file sizes recorded on the S3 is accurate enough don't you think?


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## George Webster (Apr 2, 2003)

I do all my recording with over the air broadcasts. Each show I record is on a digital channel and SD and HD seem to be all within 5.75 to 6.85 GB per hour. This means my 250 GB hard drive can only record 39.7 hours with an average of 6.3 GB per hour.

I wish Tivo would stop advertising 300 hours with the series 3 - that is only for analog and with the quality set to the worst of the four settings.


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## HDTiVo (Nov 27, 2002)

George Webster said:


> I wish Tivo would stop advertising 300 hours with the series 3 - that is only for analog and with the quality set to the worst of the four settings.


What would you replace that with? They additionally quote HD recording time based on nearly 8GB/hr. What would you like to see?


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## MickeS (Dec 26, 2002)

HDTiVo said:


> What would you replace that with? They additionally quote HD recording time based on nearly 8GB/hr. What would you like to see?


I forgot about what they use for the estimated HD recording time. Since my cable stations use at the most around 6GB/hour (and looks like it too, sadly), I guess I have around 40 hours. Nice of TiVo to estimate it based on OTA recordings and not the compressed cable recordings...


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## HDTiVo (Nov 27, 2002)

MickeS said:


> I forgot about what they use for the estimated HD recording time. Since my cable stations use at the most around 6GB/hour (and looks like it too, sadly), I guess I have around 40 hours. Nice of TiVo to estimate it based on OTA recordings and not the compressed cable recordings...


The practical effect is you get 'this will be deleted on XXX' warnings that don't occur in reality close to that date. I'm always reestimating in my head whether I really have to worry - which of course really means I still don't have big enough hard drives.


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## bkdtv (Jan 9, 2003)

Here are some average bitrates I measured:

_Off-air, Week of 9/04 - 9/09_

```
[u][b]Network[/b][/u]   [u][b]Title[/b]	                  [/u]       [u][b]Minutes[/b][/u]  [u][b]Size(Gb)[/b][/u]  [u][b]Bitrate (Mbps)[/b][/u]
[b]WRC       Tonight Show w/ Leno (9/4)       62	    7.51       16.54[/b]
WBAL      Tonight Show w/ Leno (9/4)       62	    7.03       15.48

WTTG      House (9/4)                      60       4.42       10.06
WBFF      House (9/4)                      60       4.36        9.92

WJLA      Grey's Anatomy (9/6)             61       5.31       11.89
[b]WMAR      Grey's Anatomy (9/6)             61       5.65       12.65[/b]

WUSA      U.S. Open Tennis (9/7)           60       7.24       16.48
[b]WJZ       U.S. Open Tennis (9/7)           60       8.01       18.23[/b]
WUSA      Jericho (9/7)                    60       7.11       16.18
[b]WJZ       Jericho (9/7)                    60       7.91       18.00[/b]
```
_Verizon FiOS vs OTA Comparison_

```
[u][b]Network[/b][/u]   [u][b]Title[/b]	                  [/u]       [u][b]Minutes[/b][/u]  [u][b]Size(Gb)[/b][/u]  [u][b]Bitrate (Mbps)[/b][/u]
WRC-OTA   Tonight Show w/ Leno              62	    7.51       16.54
WRC-FiOS  Tonight Show w/ Leno              62	    7.51       16.54
WUSA-OTA  Redskins at Jaguars              170     19.80       15.90
WUSA-FiOS Redskins at Jaguars              170     19.80       15.90
```
_Some programs recorded off Verizon FiOS..._

```
[u][b]Title[/b]	                  [/u]      [u][b]Channel[/b]	     [/u]  [u][b]Minutes[/b][/u]  [u][b]Size(Gb)[/b][/u]  [u][b]Bitrate (Mbps)[/b][/u]
Bronx is Burning                ESPN-HD           60	  7.50	     17.07
College Football                ESPN-HD          180     23.34       17.70
Alison Krauss & Union Station	Hdnet	         120	 15.39	     17.51
Vince Gill & Friends            Hdnet            120	 15.50	     17.64
Hunt for Red October            Hdnet Movies	 135	 17.44	     17.64
Blazing Saddles                 Hdnet Movies	 100	 12.93	     17.65
Layer Cake                      Hdnet Movies	 115	 14.85	     17.63
Heroes                          NBC-HD	          60	  7.30	     16.61
Naked Science                   NGC-HD            60	  7.56	     17.20
The Company                     TNT-HD	         120	 15.11	     17.19
The Recruit                     TNT-HD	         150	 18.96	     17.26
```


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## Revolutionary (Dec 1, 2004)

bkdtv said:


> Here are some average bitrates I measured:
> 
> _Off-air, Week of 9/04 - 9/09_
> 
> ...


Man I wish we had HDNet channels on Cox. Universal-HD is the best we get, and the bitrate is more like 7.0Mbps.


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## George Webster (Apr 2, 2003)

HDTiVo said:


> What would you replace that with? They additionally quote HD recording time based on nearly 8GB/hr. What would you like to see?


Well, I originally thought that the S3 would do 303 hours of NON HD content. That was before I discovered the setting that let you select the recording quality. There it says 52 hours on best quality but the S3 does not even provide that on NON HD content. All my OTA channels are digital and the recording quality setup screen says that those settings do not have any effect on digital channels. So I get 40 hours.

Also, I can not understand why recording a NON HD digital channel takes up the same 5.8 to 6.8 GB per hour that it takes to record a HD progrram.

HD = 1920 x 1080 = 2,073,600 pixels.

SD = 640 x 480 = 307, 200 pixels

So HD content has 6.75 times as much information in the signal. Seems to me they chose too high of a recording rate to render SD pictures and therefore wasted hard drive space.


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## aaronwt (Jan 31, 2002)

My non HD digital channels take up between 1.1 to 2.2 GB of space for an hour long show.
While the HD hour long shows take between 6GB and 8 GB of space.

I do notice that my non HD digital channel SCI Fi only takes around 1.1GB per hour on FIOS while non HD digital from Discovery or USA take around 2GB of space.


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## moyekj (Jan 24, 2006)

aaronwt said:


> I do notice that my non HD digital channel SCI Fi only takes around 1.1GB per hour on FIOS while non HD digital from Discovery or USA take around 2GB of space.


 Many of the popular SciFi shows are letterboxed (top and bottom black bars) which could help explain that...


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## aaronwt (Jan 31, 2002)

moyekj said:


> Many of the popular SciFi shows are letterboxed (top and bottom black bars) which could help explain that...


I didn't think about that. The shows from SciFi I have recorded are all in letterbox format. That would explain it. Thanks!


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## Revolutionary (Dec 1, 2004)

George Webster said:


> Well, I originally thought that the S3 would do 303 hours of NON HD content. That was before I discovered the setting that let you select the recording quality. There it says 52 hours on best quality but the S3 does not even provide that on NON HD content. All my OTA channels are digital and the recording quality setup screen says that those settings do not have any effect on digital channels. So I get 40 hours.
> 
> Also, I can not understand why recording a NON HD digital channel takes up the same 5.8 to 6.8 GB per hour that it takes to record a HD progrram.
> 
> ...


Non-HD digital _channels_, or non-HD _content_ on broadcast digital (eg, OTA-HD) channels?


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## Joybob (Oct 2, 2007)

moyekj said:


> Many of the popular SciFi shows are letterboxed (top and bottom black bars) which could help explain that...


Speaking of letterboxing. With SD letterboxed channels I end up with half my television covered in black bars. Can Tivo add more aspect settings? Zoom doesn't really seem to fix that for me.


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## moyekj (Jan 24, 2006)

Joybob said:


> Speaking of letterboxing. With SD letterboxed channels I end up with half my television covered in black bars. Can Tivo add more aspect settings? Zoom doesn't really seem to fix that for me.


 I have 1080i fixed video output set on the Tivo and Zoom mode will fill the screen just right for these letterboxed SciFi shows - i.e. it gets rid of the black bars on sides and top/bottom while preserving the 4:3 aspect ratio.


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## HDTiVo (Nov 27, 2002)

Stop watching 2.35:1 movies.


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## HDTiVo (Nov 27, 2002)

:up:

Suddenly Monday Night Football comes in at 7.2GB/hr.

Does 720p look good at 7.2/hr. Yeah baby!

On my crap CRT HDTV it looks perfect. *It looks like there is no compression!*

It actually demonstrates the superiority of CRT over excellent Sony XBR2 LCD. The source is so good the superior LCD's best doesn't equal the inferior CRT! *

Thanks to all those that made this possible!

Not only is the picture awesome, but with 1FF working on 720p content, its like being in heaven.

Note the Sony is connected to an S3. I tried both Native and 1080i fixed settings to the 1080p LCD. HDMI connections. TiVo HD at 1080i fixed to CRT.


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## George Webster (Apr 2, 2003)

aaronwt said:


> My non HD digital channels take up between 1.1 to 2.2 GB of space for an hour long show.


As I said, for me, NON HD digital channels seem to take up an average of 6.3 GB per hour. How couls yours be so different? Any idea? Is yours over the air or perhaps compressed by a cable provider?


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## Revolutionary (Dec 1, 2004)

George Webster said:


> As I said, for me, NON HD digital channels seem to take up an average of 6.3 GB per hour. How couls yours be so different? Any idea? Is yours over the air or perhaps compressed by a cable provider?


I repeat:

Non-HD digital _channels_, or non-HD _content_ on broadcast digital (eg, OTA-HD) channels?


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## moyekj (Jan 24, 2006)

George Webster said:


> As I said, for me, NON HD digital channels seem to take up an average of 6.3 GB per hour. How couls yours be so different? Any idea? Is yours over the air or perhaps compressed by a cable provider?


 For SD (480i) mpeg2 content I have never heard/seen them occupy that much space. Mine clock in ~ 2GB/hour range. That sounds more like upconverted SD being broadcast on an HD channel to me.


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## AbMagFab (Feb 5, 2001)

George Webster said:


> As I said, for me, NON HD digital channels seem to take up an average of 6.3 GB per hour. How couls yours be so different? Any idea? Is yours over the air or perhaps compressed by a cable provider?


You've posted like 5 times on this, and you're clearly confused about something. People have been trying to help out.

HD *channels*, whether it's a 16x9 show or a 4x3 show, will take up virtually the same amount of space, averaging around 6-8GB/hour.

SD *channels*, whether digital or analog, will take up roughly the same abount of space, averaging 1-2GB/hour.

At 1-2GB/hour, a 320GB hard drive can easily hold up to 300 hours on most cable systems (where it's usually closer to 1GB/hour).


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## George Webster (Apr 2, 2003)

Revolutionary said:


> I repeat:
> 
> Non-HD digital _channels_, or non-HD _content_ on broadcast digital (eg, OTA-HD) channels?


Digital channels all, NON HD content.


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## bkdtv (Jan 9, 2003)

George Webster said:


> Digital channels all, NON HD content.


SD content on HD channels is upconverted to HD resolution (1280x720 or 1920x1080) and thus requires comparable bandwidth to true high-def.

Said a different way, everything recorded off a HD channel requires about the same amount of bandwidth. The source doesn't matter.


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## MickeS (Dec 26, 2002)

George Webster said:


> Digital channels all, NON HD content.


That still doesn't say much, unfortunately. 

Non-HD content on a digital HD channel will take up roughly the same space as HD content on that channel. That seems to be what you're talking about.
For example, recording a non-HD daytime soap on your local ABC HD affiliate (cable or OTA) will take up roughly the same space as recording the HD "Grey's Anatomy" on that same channel later at night, probably around 6-8GB depending on compression.

SD content on a digital SD channel will take up much less space. Recording the same daytime soap mentioned above from the digital SoapNet channel would take up maybe 1-2 GB - and they would look about the same.


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## George Webster (Apr 2, 2003)

AbMagFab said:


> You've posted like 5 times on this, and you're clearly confused about something. People have been trying to help out.


 Perhaps I am confused about something. Here is an attempt to fill in the blanks.



AbMagFab said:


> SD *channels*, whether digital or analog, will take up roughly the same amount of space, averaging 1-2GB/hour.


 Then how do you explain the following?

I just checked my Tivo. Two shows, both recorded on 3.1, a digital channel. (CBS for me)

CSI: NY labeled as HD and it takes up 5.90 GB
Oprah Winfrey NOT labeled as HD yet it takes up 5.87 GB

Both programs exactly one hour long.


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## George Webster (Apr 2, 2003)

bkdtv said:


> SD content on HD channels is up converted to HD resolution (1280x720 or 1920x1080) and thus requires comparable bandwidth to true high-def.


 I did not know that TV stations are up converting. 


bkdtv said:


> Said a different way, everything recorded off a HD channel requires about the same amount of bandwidth. The source doesn't matter.


 So then, if the up converted signal looks substantially better than the original then I should notice that up converted programs look almost as good as true high definition? Please understand that my series 3 has never been hooked up to a high definition set and thus far I have not ever had a high definition set.

If the above is true then I do not mind that the Tivo is using more hard drive space because the difference between true HD shows and up converted shows might be only a slight difference. That would be cool if it is true.


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## Revolutionary (Dec 1, 2004)

George Webster said:


> Perhaps I am confused about something. Here is an attempt to fill in the blanks.
> 
> Then how do you explain the following?
> 
> ...


You have proven what he stated.

It was the same channel.

What the Tivo records is the raw digital signal as it is transmitted by the broadcaster. It directly captures what is called the "transport stream", or .ts when you do it via HTPC.

With an HD-channel, such as your network locals, the broadcast is ALWAYS a HiDef transport stream, even if the image that the stream contains is not a HiDef image.

As an example, when Oprah films her show, the image is 4:3, 640x480. When it is broadcast on cable, it is broadcast at 4:3, 640x480. But when your local ABC station gets the feed, they "wrap" it in a hi-def transport stream of 16:9, 1920x1080. Essentially they blow the image up to 1440x1080, and then they add in the black windowboxes on the sides (each at 240x1080). The Tivo captures it as if there was a 16:9, 1920x1080 image in every frame, because there effectively is -- the black bars count. Thus, no matter what you record off the HD channel, the Tivo will always just capture the transport stream; and the transport stream from a given station is (or should be) static. They are broadcasting the stream at pretty much the same bitrate all the time (there can be slight variations with individual broadcasters).

Thus, if you want to reduce the size of your recordings that are _not_ HD content (ie, the guide says HD), you should record them from another version of that channel (either the broadcast or cable analog version).

editted: originally had the numbers reflect a 720p station, but then realized you said it was a CBS affiliate, which means its actually 1080i.


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## George Webster (Apr 2, 2003)

MickeS said:


> Non-HD content on a digital HD channel will take up roughly the same space as HD content on that channel. That seems to be what you're talking about.


 Yes, that is what I am talking about. Now I think I am beginning to understand.



MickeS said:


> SD content on a digital SD channel will take up much less space.


 Now I think I get it. My channel 3.1 = all programs at about the same hard drive usage because of up converting.

My channel 3.2 = SD = no up converting = much less hard drive space used.

That about right?


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## MickeS (Dec 26, 2002)

George Webster said:


> Now I think I get it. My channel 3.1 = all programs at about the same hard drive usage because of up converting.
> 
> My channel 3.2 = SD = no up converting = much less hard drive space used.
> 
> That about right?


You got it.


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## George Webster (Apr 2, 2003)

Revolutionary said:


> What the Tivo records is the raw digital signal as it is transmitted by the broadcaster. It directly captures what is called the "transport stream". With an HD-channel, such as your network locals, the broadcast is ALWAYS a HiDef transport stream, even if the image that the stream contains is not a HiDef image.


 I did not know that, thank you for taking the time to explain it. I think things would have been more clear to me if i owned a high definition TV. I will purchase soon.

They are broadcasting the stream at pretty much the same bit rate all the time. Got it. Thanks again

I do not really want to reduce the size of my recordings if the other recordings are up converted to having more pixels and widerscreen. You seem to be agreeing with what another poster recently wrote about the up converting. Thanks again.

I am sure things will become much more clear to me once I finally get an HDTV. This 4/3 crap with a series 3 is driving me crazy. Thinking of getting the 65 inch Toshiba 65hm167- went and saw it recently. At $2199 it is a killer set for the price. Thanks.


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## George Webster (Apr 2, 2003)

MickeS said:


> You got it.


Thanks for confirming.


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## AbMagFab (Feb 5, 2001)

George Webster said:


> Perhaps I am confused about something. Here is an attempt to fill in the blanks.
> 
> Then how do you explain the following?
> 
> ...


You don't understand the difference between "digital" and SD vs. HD. Forget about analog channels for now. I'll assume you have Comcast - every channel above 100 is *digital*, but only a few are HD, the rest are SD.

*Channels* are either HD or SD. 3.1 is, by definition, an HD channel. So anything you record off 3.1 is HD, whether it's visually 4x3 or 16x9, and will take up pretty much the same space, about 6-8GB/hour.

Channel 101 is SD, also digital, but it always 4x3, and will generally take 1-2GB/hour.

Got it?

(I guess I see why BB, CC, and everyone else keeps saying people don't understand HD.)


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## AbMagFab (Feb 5, 2001)

George Webster said:


> I did not know that TV stations are up converting.
> So then, if the up converted signal looks substantially better than the original then I should notice that up converted programs look almost as good as true high definition? Please understand that my series 3 has never been hooked up to a high definition set and thus far I have not ever had a high definition set.
> 
> If the above is true then I do not mind that the Tivo is using more hard drive space because the difference between true HD shows and up converted shows might be only a slight difference. That would be cool if it is true.


Not to confuse things, but the "upconverted" signal might not look too much better than the SD version of the show. First, it will be 4x3, so you're not gaining any picture space. Second, unless they feed from their higher quality source, it might look very similar to the SD feed.

On a non-HD set, there's even less chance that it will be noticably better (to most people).

Some shows, like Survivor, do look better. Do they look 4x better (for the disk space factor)? Probably not. But if you have added disk space to your Tivo, it might be worth it to you to record the HD channel version of the SD show to get the slightly better picture.

All up to you, but don't expect HD quality SD shows.


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## mikeyts (Jul 10, 2004)

bkdtv said:


> Here are some average bitrates I measured:
> 
> _Off-air, Week of 9/04 - 9/09_
> 
> ...


How are you calculating bit rate? Is it based on the recording sizes? No matter what assumption I make, I get different numbers than you. Assuming "GB" means 2-to-the-30th 8-bit-bytes, I've always used:size-in-GB * 8590-million-bits-per-GB = total-million-bits-in-recording / seconds = average-million-bits-per-second​For instance, I get 17.34 Mbps average for Leno on WRC (7.51GB for 62 minutes). Ah--I think I see what you're doing. You're assuming 2-to-the-30th 8-bit bytes in a GB, but the "megabits" that you're quoting are 2-to-the-20th, so you're using something equivalent to:size-in-GB * 8192-megabits-per-GB = total-megabits-in-recording / seconds = average-megabits-per-second​In transmission rates the prefixes "kilo", "mega" and "giga" are always powers of 10 (10-to-the-3rd for "kilo", 10-to-the-6th for "mega" and 10-to-the-9th for "giga"). Now, go back and do it right !


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## moyekj (Jan 24, 2006)

mikeyts said:


> How are you calculating bit rate? Is it based on the recording sizes? No matter what assumption I make, I get different numbers than you. Assuming "GB" means 2-to-the-30th 8-bit-bytes, I've always used:size-in-GB * 8590-million-bits-per-GB = total-million-bits-in-recording / seconds = average-million-bits-per-second​For instance, I get 17.34 Mbps average for Leno on WRC (7.51GB for 62 minutes). Ah--I think I see what you're doing. You're assuming 2-to-the-30th 8-bit bytes in a GB, but the "megabits" that you're quoting are 2-to-the-20th, so you're using something equivalent to:size-in-GB * 8192-megabits-per-GB = total-megabits-in-recording / seconds = average-megabits-per-second​In transmission rates the prefixes "kilo", "mega" and "giga" are always powers of 10 (10-to-the-3rd for "kilo", 10-to-the-6th for "mega" and 10-to-the-9th for "giga"). Now, go back and do it right !


 Calculations are correct. For example, the 7.51 GB recording that is 62 minutes long:
7.51 GB = 7.51 * 1024 MB/GB * 8 bits/B = 61,521.92 Mbits
So rate is 61,521.92 Mbits / (62*60 secs) = 16.54 Mbits/sec

So key things to remember:
1GB = 1Giga Byte = 1024 MB = 1024 Mega Bytes
1 B = 1 Byte = 8 bits
Mbps = Mega bits per second


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## mikeyts (Jul 10, 2004)

moyekj said:


> Calculations are correct. For example, the 7.51 GB recording that is 62 minutes long:
> 7.51 GB = 7.51 * 1024 MB/GB * 8 bits/B = 61,521.92 Mbits
> So rate is 61,521.92 Mbits / (62*60 secs) = 16.54 Mbits/sec


Though there are 2-to-the-10th (1024) megabytes per gigabyte, a _megabit_, in terms of transmission rates, is not 2-to-the-20th bits, it's 10-to-the-9th. "Mbps" is millions of a bits per second (the "M" does stand for "mega", but in this case the prefix, "mega" means "million", as it does in "megaton" or "megawatt"). Likewise, "Kbps" means thousands-of-bits-per-second, not 1024s-of-bits-per-second".

When you discuss data rates in terms of _bytes_ then "kilo", "mega" and "giga" revert to their powers-of-2 meaning. 1 "MBps" is 1048576 bytes per second (8.3886 Mbps), not one million bytes per second.

Google "Mbps" and check any online definition you please.

EDIT: Sorry--right after I typed that last statement I did google it and found that the definition at wisegeek.com is wrong. I also found a couple of others which, though correct, are sketchy and slightly confusing. For a cogent discussion, check the Wikipedia entry for "Megabit per second". It speaks about the IEC's attempt to get rid of the confusion by adding a bunch of ugly prefixes to the language which refer to powers of 2: kibi, mebi and gibi. When referring to 10-to-the-9th bits-per-second, you'd use the common Mbps or Mbit/s; for 2-to-the-20th bits-per-second you'd use Mibit/s. Ugh.


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## moyekj (Jan 24, 2006)

The rates given are in Mega bits per second, not Mega Bytes per second.
So if you agree that:
1GB = 1024 MB = 8*1024 Mbits = 8*1024*1e6 bits
Then the rate is (total # bits) / (total secs) and the resulting Mbps means Million bits per second
Here's Wikepedia definition of Mbps:
http://en.wikipedia.org/wiki/Mbps


> A megabit per second (abbreviated as Mbit/s, Mbps, or mbps) is a unit of data transfer rate equal to 1,000,000 bits per second


EDIT: I guess you found the same definition I found and I think you agree the posted rates are correct now


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## mikeyts (Jul 10, 2004)

moyekj said:


> The rates given are in Mega bits per second, not Mega Bytes per second.
> So if you agree that:
> 1GB = 1024 MB = 8*1024 Mbits = 8*1024*1e6 bits
> Then the rate is (total # bits) / (total secs) and the resulting Mbps means Million bits per second
> ...


Your problem is that 1024 MB does not equal 8*1024 Mbits, if you define a 1 Mbit as 1,000,000 bits. 8 * 1024 million bits is 8,192,000,000 bits. 1 MB = 1,048,576 bytes = 8,388.608 bits. 1024 MB (aka 1 GB) = 8,589,934,592 bits or 8,589.93 million bits, which I round up to 8,590 for purposes of bit rate calculations (the difference is so small that all the error will be in digits below 2 decimal places).

The posted calculations are wrong because they assume that the "Mb" in "Mbps" is 1,048,576 bits and not 1,000,000 bits.


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## moyekj (Jan 24, 2006)

I think you're right: I guess the posted rates are technically Mebibits per second = 1,048,576 bits per second but as you say the difference is not that far off. For comparison reasons since they are all calculated the same way it's the relative differences that matter anyway not the absolute numbers.


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## mikeyts (Jul 10, 2004)

The difference between Mbit/s and Mibit/s is 4.63% (Mbits/s higher than Mibits/s). Seems like a substantial difference to me, but your mileage may vary. I'd go back and correct the numbers, but do as you will. Just be aware that people used to bandering about transmission rates as a potential measure of PQ will be confused should they enter this discussion unawares.


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## mattack (Apr 9, 2001)

George Webster said:


> I wish Tivo would stop advertising 300 hours with the series 3 - that is only for analog and with the quality set to the worst of the four settings.


Everything advertises with the best possible look at the product. For a Tivo, recording capacity is the gauge to use. Plus, they do mention the different rates for HD hours vs analog hours.

(I use my S3 for only analog recording at basic quality, with a FEW manual QAM recordings so far..)


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## mattack (Apr 9, 2001)

Revolutionary said:


> With an HD-channel, such as your network locals, the broadcast is ALWAYS a HiDef transport stream, even if the image that the stream contains is not a HiDef image.


But wait, some channels, e.g. like KQED in the SF Bay Area, show HD in prime time, but multicast during the day.

The multicast recordings are lower resolution/bandwidth than the HD streams.. correct?
(must be, since they're all taken out of the same 6 MHz bandwidth per station...??)


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## moyekj (Jan 24, 2006)

mikeyts said:


> The difference between Mbit/s and Mibit/s is 4.63% (Mbits/s higher than Mibits/s). Seems like a substantial difference to me, but your mileage may vary. I'd go back and correct the numbers, but do as you will. Just be aware that people used to bandering about transmission rates as a potential measure of PQ will be confused should they enter this discussion unawares.


 To avoid confusion in the future perhaps it's best to calculate as GB/hour instead which makes the calculation much cleaner and less room for confusion. My original post merely compares total file sizes, not bitrates, so there should be not confusion there. It's up to bkdtv if he wants to correct or clarify his post.


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## mikeyts (Jul 10, 2004)

moyekj said:


> To avoid confusion in the future perhaps it's best to calculate as GB/hour instead which makes the calculation much cleaner and less room for confusion. My original post merely compares total file sizes, not bitrates, so there should be not confusion there. It's up to bkdtv if he wants to correct or clarify his post.


That's highly unconventional, which is why bkdtv calculated transmission bit rates in his post. It's how people talk about bandwidth, in terms of how many symbols are being transmitted on average per second, not per hour. Just remember that the convention is a million-bits-per-second, not 2-to-the-20th-"mebibits"-per-second. It's easy, as is the calculation.

There are constants that many of us are familar with that can be used for comparison. For instance, the maximum amount of information that can be transmitted over-the-air in an 8 VSB modulated stream is 19.39 Mbps (that's 8.126 GB/hour, but no one thinks of it that way); a QAM 256 carrier can hold exactly twice as much in the same band (6 MHz), at 38.8 Mbps. (QAM uses far less bandwidth for noise rejection, and broadband cable is a far less noisy environment than an open-air microwave transmission).

My original post was an appeal to bkdtv to modify his numbers, which is up to him. Don't worry about it.


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