Cable TV Sound Options

[FatStrat85]

I have Time Warner Digital/HD cable. The cable box gives me the option to output either Dolby or PCM over the coaxial digital connection to my receiver. Some of the HD channels output surround sound at 384kbps or 448kbps at 48kHz using Dolby. The regular digital (non-HD) channels only output in 2-channel stereo at 128kbps or 192kbps at 48kHz using Dolby. Also, even some of the HD channels output in 2.0, but still at the higher bitrate of 384kbps or 448kbps. If I switch to PCM mode, there is obviously no bitrate displayed, but it's still at 48kHz.

My question is regarding the channels that output in 2.0. Would the sound quality be better using PCM instead of Dolby or does it just send a PCM version of the same compressed sound? Also, when Dolby HD channels are sending 2-channel audio, since the bitrate stays the same, does that mean each channel is less compressed than the multi-channel 5.1 (384/2 instead of 384/6)?

I'm mostly interested in the 40 dedicated music channels. I'm wondering if 2.0 PCM is higher quality than 2.0 Dolby at 192kbps.

 

[Adam]

I don't know the answer (but I'm guessing that someone, such as MDS, does).

My questions to you are: Do the two settings sound any different to you? If so, do you prefer one over the other?

EDIT: BTW, I'm impressed on how you delve into the details (this is the third thread that I've seen where you pose similar questions). It keeps me learning new things.

 

[FatStrat85]

That would be the practical way to answer my question. However, TV doesn't offer reference recordings that I am familiar with to perform listening tests, so it is difficult. I have not spent enough time listening yet to form any sort of conclusion. So far, they sound slightly different, but it may just be a matter of a difference in output level. I'm not sure yet. I suppose you could conclude that since neither mode immediately sounds significantly better or worse than the other, the point is moot. However, I am still curious and would like to obtain the best sound quality possible from my system. I plan on eventually recording something that I am familiar with using my DVR and playing them back repeatedly in both modes to see if I can find a difference, but in the mean time, I'd still appreciate any input from the experts here.

EDIT: Regarding your edit, Adam; I try to be as detailed as possible so you have all the information you need to answer my question properly. Thank you for noticing. I figure that no one will want to spend the time answering my dumb questions if I don't at least put in the time to formulating and expressing them well. I'm glad you are able to take something away from my questions.

 

[chadnliz]

I have same providor, box and connections and run all Digital............its the same but you will get best sound possible for HD.

 

[MDS]

My question is regarding the channels that output in 2.0. Would the sound quality be better using PCM instead of Dolby or does it just send a PCM version of the same compressed sound? Also, when Dolby HD channels are sending 2-channel audio, since the bitrate stays the same, does that mean each channel is less compressed than the multi-channel 5.1 (384/2 instead of 384/6)?

Setting the box to output Dolby Digital vs PCM is the same as the analogous settings on a DVD player with the exception that the box will not convert analog audio to PCM and output over the digital out like a DVD player will (at least my Scientific Atlanta 3100HD box will not).

If you set the output of the box to Dolby Digital:
- If the source is DD 5.1 or DD 2.0, it will just send the bitstream untouched to the receiver and the receiver will do the decoding - the same as setting a DVD player to bitstream. If the source is PCM it will also just pass thru.

If you set the output of the box to PCM (mine just calls the setting 'other'):
- If the source is DD 5.1 or DD 2.0, the box will decode and downmix if necessary to 2 channel PCM.
- If the source is PCM, it will just pass thru because it is already PCM.

Regardless of the digital out setting, if the source is analog audio, you will hear nothing unless you also hook up analog audio cables and have the receiver set to Auto so it can automatically switch between digital and analog.

Don't get too hung up on the bitrates. If the bitrate for DD 5.1 is 384 kbps, it does not mean that each channel uses 384/6 kbps. For any given second of audio, each channel does not necessarily have the same amount of data. If it did, then the surrounds would be active at all times.

 

[FatStrat85]

If the source is DD 5.1 or DD 2.0...

If the source is PCM...

How do I know what the "source" is?

Regardless of the digital out setting, if the source is analog audio, you will hear nothing unless you also hook up analog audio cables and have the receiver set to Auto so it can automatically switch between digital and analog.

I don't think the source could be analog because I always hear something. Is what you are saying that it won't make a difference either way I set it? 2-channel PCM won't sound any better than 2-channel Dolby, regardless of the source, or vice versa? I thought that perhaps setting it to Dolby would create compression that could be avoided by setting it to PCM.

 

[LoveElectronics]

Setting the box to output Dolby Digital vs PCM is the same as the analogous settings on a DVD player with the exception that the box will not convert analog audio to PCM and output over the digital out like a DVD player will (at least my Scientific Atlanta 3100HD box will not).

If you set the output of the box to Dolby Digital:
- If the source is DD 5.1 or DD 2.0, it will just send the bitstream untouched to the receiver and the receiver will do the decoding - the same as setting a DVD player to bitstream. If the source is PCM it will also just pass thru.

If you set the output of the box to PCM (mine just calls the setting 'other'):
- If the source is DD 5.1 or DD 2.0, the box will decode and downmix if necessary to 2 channel PCM.
- If the source is PCM, it will just pass thru because it is already PCM.

Regardless of the digital out setting, if the source is analog audio, you will hear nothing unless you also hook up analog audio cables and have the receiver set to Auto so it can automatically switch between digital and analog.

Don't get too hung up on the bitrates. If the bitrate for DD 5.1 is 384 kbps, it does not mean that each channel uses 384/6 kbps. For any given second of audio, each channel does not necessarily have the same amount of data. If it did, then the surrounds would be active at all times.

They should have you as the GOD of Audioholics

 

[Adam]

They should have you as the GOD of Audioholics

Oh, great ones...have mercy upon him. He meant demigod. Really. No need to spite LoveE for a mere typo, right?

[The gods within Mount AH begin to contemplate...and watch another movie.]

 

[MDS]

How do I know what the "source" is?

You must know because you said some channels are DD 5.1, some are DD 2.0, and some are PCM. I would assume you get that info from the receiver's display. The source is your cable box but I just meant that the audio format from each channel can vary and what gets output by the cable box depends on the Dolby Digital vs PCM setting and the format of the audio that is present.

I don't think the source could be analog because I always hear something.

My cable system must be different than yours. The channels below 70 broadcast analog audio only. That was the first lesson I learned about 'digital cable' when I first signed up for it around the year 2000. I got a new receiver, connected the cable box with a digital coax cable, turned on the TV, and got no sound. I have to use both analog and digital connections and with the receiver set to Auto, it automatically switches between digital and analog. If you have alll digital audio on every channel, then you won't have to use analog connections at all.

Is what you are saying that it won't make a difference either way I set it? 2-channel PCM won't sound any better than 2-channel Dolby, regardless of the source, or vice versa? I thought that perhaps setting it to Dolby would create compression that could be avoided by setting it to PCM.

I'm just telling you how it processes the incoming audio. SQ differences are in the ear of the listener.

DD is a compressed format but setting the box to PCM will not eliminate the compression, it just decodes it to PCM (and if it were 5.1, also downmix the 5.1 to 2.0). You can't put back what was taken away during the encoding process.

If your receiver couldn't decode DD, then you'd have to use the PCM setting. Since it can, you should use the DD setting so that the receiver can do the decoding. When the receiver gets the raw bitstream it can do things other than simple decoding - like apply late night mode, cinema filter, any post processing routines like THX, etc.

 

[FatStrat85]

Thanks. All of my cable channels have digital audio. I will use Dolby instead of PCM based on your recomendation.