TiVo : PCM or Dolby

[steveh]

All you're really saying is that you prefer the sound of the Dolby Digital decoder in the TiVo to the one in your AV amp. Nothing wrong with that but that's all it is.

[mm7000gb]

steveh: If I am receiving PCM from the tivo, and using PCM for 2.0 Chan stereo, I'm not using Dolby Digital.
REMEMBER: The PCM option is for equipment that MAY NOT BE ABLE TO DECODE Dolby Digital.
Did you really think this through?

[steveh]

The transport stream that your TiVo receives from Virgin for an HD channel only contains Dolby Digital audio*. It does not contain a separate PCM audio stream. If you choose the PCM option in the TiVo settings then the TiVo will use its internal Dolby Digital decoder to convert that Dolby stream into a PCM stream to send to your AV amp. If you choose the Dolby option then it will instead pass through the Dolby Digital stream to your AV amp to decode instead.

(* Except for those HD channels like BBC News that aren't in Dolby and like the SD channels use MPEG Layer 2 audio instead, which the TiVo will always convert to PCM before sending to your amp. Some also have an additional audio description stream that may not be in Dolby.)

[mm7000gb]

steveh: So (only) BBC HD channels and all SD channels are not Dolby Digital, hence the better sound quality using the PCM stream imo.
That's still a lot of channels though, even if you are correct about all the other HD channel streams.
Probably at least half of all available channels on the tivo are not Dolby Digital then.

[Stephen]

The ITV HD channels are also not DD I believe.

[steveh]

Originally Posted by mm7000gb

steveh: So (only) BBC HD channels and all SD channels are not Dolby Digital, hence the better sound quality using the PCM stream imo.

Just to be completely clear:

1. The PCM audio that you get out of your TiVo is always converted by the box from a compressed format.

2. There is no PCM audio delivered to set top boxes through Virgin's cable TV network.

3. Dolby Digital decoders can sound different in different devices for reasons to do with cost, age, settings and the kind of device they are. That's what you're hearing.

4. The BBC News Channel in HD does not use Dolby Digital, the other BBC HD channels do.

[mm7000gb]

Is it possible that the PCM stream I get from the tivo box that is from a compressed format is of better quality than the Dolby Digital stream that is also from the same compressed format?
Also, on channels that are not Dolby Digital (Like the BBC HD News Channel) Why does the selection of either PCM or DD from the tivo sound different at the AV receiver, if there is no DD option what is the DD selection at the tivo streaming that sounds different to the PCM option?

[steveh]

Originally Posted by mm7000gb

Is it possible that the PCM stream I get from the tivo box that is from a compressed format is of better quality than the Dolby Digital stream that is also from the same compressed format?
Also, on channels that are not Dolby Digital (Like the BBC HD News Channel) Why does the selection of either PCM or DD from the tivo sound different at the AV receiver, if there is no DD option what is the DD selection at the tivo streaming that sounds different to the PCM option?

I don't have any knowledge of the TiVo internals but I would have thought it very unlikely there was any difference for non-Dolby Digital (i.e. MPEG Audio Layer 2) channels depending on the audio setting.

The chain is like this...

TiVo set to PCM output:

Channel in MP2 -> TiVo converts to PCM -> Your amp plays PCM
Channel in DD -> TiVo converts to PCM -> Your amp plays PCM

Tivo set to Dolby Digital output:

Channel in MP2 -> TiVo converts to PCM -> Your amp plays PCM
Channel in DD -> TiVo passes DD straight through -> Your amp decodes DD

Dolby Digital gives higher quality than MPEG Audio Layer 2 assuming the same bitrate. However, as I said before the TV companies may do different audio processing on Dolby and non-Dolby channels so they won't sound the same. An average user will be more likely to hear the differences in processing than any differences between the codecs.