# Any Reason You'd Want To Turn Off Dolby Digital?



## MikeekiM (Oct 1, 2006)

I have an HR20-700S that is distributed to two different displays... One display uses the internal TV speakers and has no Dolby Digital capabilities. The other display uses a Dolby Digital receiver...

The instructions in the set up screens say to turn DD off if not using a DD capable setup, and turn it on if you do... Since I have both, I have turned DD on... Is there any disadvantage to this when I watch in the non-DD capable room?


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## cmtar (Nov 16, 2005)

I don't think there is but I may be wrong, mine is always on


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## spartanstew (Nov 16, 2005)

No disadvantage.

Both of my HR21's are distributed to 6 other TV's, none of which utilize a separate sound system. Both HR21's have DD turned on, because the main set-up utilizes a home theater system.

I hear no adverse effects.


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## bhelton71 (Mar 8, 2007)

Maybe if you had a tv with digital audio inputs, but no DD decoder - you would probably be greeted with a high pitched squeal. I don't know that any such TV exists ?

I would think most people would use the 2 channel audio anyway to connect directly to a TV without a DD decoder.


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## tonyd79 (Jul 24, 2006)

Isn't this a matter of recording size? If you turn DD off, doesn't it not store the DD information? I could be way off but I thought that is what it was about.


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## Stuart Sweet (Jun 19, 2006)

If you're not using a home theater, getting all the sound from your TV, turning DD off will probably make the volume seem more consistent across channels.


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## Steve (Aug 22, 2006)

tonyd79 said:


> Isn't this a matter of recording size? If you turn DD off, doesn't it not store the DD information? I could be way off but I thought that is what it was about.


I think in theory you are correct, but in practice, the difference between DD and PCM stereo is not a huge % of the overall recording file size.

E.g., I just looked at a DVD of the movie _Pleasantville _(MPEG-2).

Video with DD is 4997 MB
Video with 2 Channel Stereo is 4759 MB

Only a 5% difference in filesize.

Assuming MPEG-4 is compressed about 30% more efficiently than MPEG-2:

Video with DD maybe 3622 MB
Video with Stereo maybe 3384 MB.

Only a 6.5% difference in filesize.

/steve


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## jaguar325 (Jan 2, 2006)

Some programs are broadcast in DD audio (usually newer and HD format shows), others are broadcast in good old-fashioned 2-channel (usually older, SD format shows). Some devices (older A/V receivers and certain TVs) cannot automatically switch between the two forms of audio or can't convert the digital output of your DTV box. Because of this, people can have trouble with audio (the symptoms usually being able to hear some programs and not others). Turning DD off, means that the DTV box will only put out 2-channel, analog, "PCM" output - this normally fixes the problem but it means no DD.. not a big deal with TV speakers but it can make a noticeable difference with a home theatre setup to take advantage of 5+ discrete channels because switching to PCM means using the older "surround sound" audio fields.


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## Earl Bonovich (Nov 15, 2005)

I typically turn it off... when we have parties going on in the house, and it is just general TV viewing... (not necessarily a movie or something).

That the way the audio goes out over most of the 7 speakers... vs... the "correct" ones.

Makes it easier for those all over the room to hear the show while conversations are going on.


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## gcisko (Sep 27, 2006)

Steve said:


> I think in theory you are correct, but in practice, the difference between DD and PCM stereo is not a huge % of the overall recording file size.


My TV is DD capable and sends DD to my home theater via optical out when the signal is DD via OTA. However when I use my HR20, it goes to the TV via HDMI and then the optical out is always PCM 48. I do not hear much difference with the seperation of the channels versus DD. Does this seem correct or am I dreaming?


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## Steve (Aug 22, 2006)

gcisko said:


> My TV is DD capable and sends DD to my home theater via optical out when the signal is DD via OTA. However when I use my HR20, it goes to the TV via HDMI and
> then the optical out is always PCM 48. I do not hear much difference with the seperation of the channels versus DD. Does this seem correct or am I dreaming?


Sounds like your TV is properly downmixing 5.1 to stereo, per the DD spec.

Here's what the Dolby website says on the subject:

_*6. Can I hear 5.1-channel Dolby Digital programs over a regular
stereo, Dolby Pro Logic, or Dolby Pro Logic II system?*

Yes. All Dolby Digital decoders, whether 5.1-channel or two-channel, have a unique feature called "downmixing" that assures full compatibility with any playback system. At your choice, the decoder will create "on the fly" from 5.1-channel programs a two-channel mix encoded in Dolby Surround for playback over a home theater system with Dolby Pro Logic decoding; a two-channel stereo mix for regular stereo and headphone playback; a mono mix for playback over a mono TV set; and even five independent full-range channels through a Dolby Pro Logic II system. A system equipped with Dolby Pro Logic II can decode five full-range channels from the two-channel Dolby Digital downmix source.

Thus, producers of 5.1-channel programs need only provide the one 5.1-channel mix. The decoder in the playback system does the rest, automatically making the signal conform to the particular playback circumstances._

/steve


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## bhelton71 (Mar 8, 2007)

Steve said:


> I think in theory you are correct, but in practice, the difference between DD and PCM stereo is not a huge % of the overall recording file size.
> 
> E.g., I just looked at a DVD of the movie _Pleasantville _(MPEG-2).
> 
> ...


Actually I expect they wouldn't do anything to the audio - when you convert, you would presumably strip the AC3 intact and only the video stream would be transcoded to MPEG4. If they moved to AAC you might get some size improvements.


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## Steve (Aug 22, 2006)

bhelton71 said:


> Actually I expect they wouldn't do anything to the audio - when you convert, you would presumably strip the AC3 intact and only the video stream would be transcoded to MPEG4. If they moved to AAC you might get some size improvements.


What if you wanted to output PCM audio to a non-DD receiver or TV (if there is such a thing these days)? Is the HR2x doing the downsampling and conversion in that case?

If so, then I agree. No disk storage size savings to be realized at all. In this case, the DD setting in preferences is simply an output mode instruction for the HR2x. /steve


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## bhelton71 (Mar 8, 2007)

Steve said:


> What if you wanted to output PCM audio to a non-DD receiver or TV (if there is such a thing these days)? Is the HR2x doing the downsampling and conversion in that case?
> 
> If so, then I agree. No disk storage size savings to be realized at all. In this case, the DD setting in preferences is simply an output mode instruction for the HR2x. /steve


Sorry - I was only referring to the MPEG4 re-encoding - not anything internal to the box.

Apparently the BCM7401 (the HR21 ?) does not appear to be able to downmix from 3/2 to 2/0 - on the block diagram it appears 3/2 is a passthru or is decodes to 2 channel PCM and ships over digital or fed into a single dac to feed the stereo outs.

http://www.broadcom.com/collateral/pb/7401-PB04-R.pdf

I agree with you - they are just using it as a 'passthru' Y/N toggle.


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## Steve (Aug 22, 2006)

bhelton71 said:


> Sorry - I was only referring to the MPEG4 re-encoding - not anything internal to the box.
> 
> Apparently the BCM7401 (the HR21 ?) does not appear to be able to downmix from 3/2 to 2/0 - on the block diagram it appears 3/2 is a passthru or is decodes to 2 channel PCM and ships over digital or fed into a single dac to feed the stereo outs.
> 
> ...


Yup. That PDF was a nice find.

So in summary, there will be no disk space savings with DD "off", because it appears the HR2x is always recording 5.1 when it's present. The HR2x then either outputs either 5.1 channels or down-mixed stereo, depending on the DD SETUP option selected by the viewer.

/steve


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## jaguar325 (Jan 2, 2006)

Steve said:


> Yup. That PDF was a nice find.
> 
> So in summary, there will be no disk space savings with DD "off", because it appears the HR2x is always recording 5.1 when it's present. The HR2x then either outputs either 5.1 channels or down-mixed stereo, depending on the DD SETUP option selected by the viewer.
> 
> /steve


If you turn DD back on, it will play a previously recorded show (one recorded during the time you had DD off) in full DD. Just like HD video, it records the show in whatever format it is being broadcast and does the conversion at time of playback. Hence, no disk savings.


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