PCM conversion

[RickC3C4]

I don't know if "Receivers" is the right category for this subject but since my receiver is the unit in question I thought I would throw it out here. My question is about PCM (Pulse Code Modulation). I'm running an all digital signal so why does the "PCM" indicator keep coming on? When I'm watching an HD channel or program the receiver displays a Dolby Digital signal is being received and processed. But when I'm watching a non-HD channel or program the receiver displays the signal is being processed through Dolby Pro logic II and the PCM indicator also lights up.

Now I know the signal was originally broadcast as an analog signal but I'm using DirecTV so it's obviously been converted and is being received as a digital signal. More importantly I'm only using digital optical cables/connections so why (and where) is the signal being converted from analog to digital making it a PCM signal?

 

[j_garcia]

You have two different questions there. What is being broadcast and why am I getting PCM via digital? PCM IS a digital signal (all redbook CDs are PCM for example), and via optical and coaxial digital connections, it is effectively limited to stereo only (dts-CDs being an exception). As to why the receiver reads PCM, that simply means for that particular broadcast, they are not transmitting a Dolby Digital signal (which may be anywhere from 1 to 8 channels). When the receiver detects a PCM signal, if left on "auto detect", the receiver will generally default to ProLogic/II/IIx by default. Just because you have a digital connection does not mean you are receiving a multichannel bitstream broadcast and everything is not "converted" to Dolby Digital or DTS.

 

[RickC3C4]

I do understand that unless a signal is broadcast as a Dolby Digital signal it will not be displayed or processed as a Dolby Digital signal and will by default through the autodetect feature be processed as a Pro Logic/II/IIx signal. Admittedly I'm not fully knowledgeable about the PCM signal but it's my understanding that a PCM signal is what is created through the analog to digital converter. This is where the crux of my question comes from, as explained there should be no analog signal to convert to digital so why am I getting the PCM displayed? I'm not saying my receivers doing anything wrong I'm just trying to understand the process.

 

[MichaelJHuman]

Your TV needs a way to send non HD audio over it's optical cable. The audio is either mono or stereo. The simplest way to do that is to use PCM which is part of the S/PDIF standard.

PCM is simply a stream of numbers which you can think of as representing the voltage of the signal.

 

[Nick250]

Starting with the Direct TV box, what wiring configuration are you using?

Nick

 

[RickC3C4]

Your TV needs a way to send non HD audio over it's optical cable. The audio is either mono or stereo. The simplest way to do that is to use PCM which is part of the S/PDIF standard.

PCM is simply a stream of numbers which you can think of as representing the voltage of the signal.

MichaelJHuman,

True, and when it is not receiving a true surround sound signal it usually switches and displays "stereo". I've been doing a little research and understand what PCM is I'm just still not clear on when it is or should be used. Plus the fact that I do not have PCM conversion turned on anywhere in the setup configuration continues to baffle me. Like there is a option to use Dolby or you can select to convert Dolby to PCM, why would I want to do that?

 

[RickC3C4]

Starting with the Direct TV box, what wiring configuration are you using?

Nick

Nick250,

I don't know how involved you want to get here, because my receiver does not have HDMI connections I'm running all my video through component connections and all my audio through digital optical connections. So I come out of the DirecTV box and into the AV receiver with component video cables and digital optical cables. Likewise I'm running the same wiring configuration out of my DVD/SACD player as well as my DVD changers. Of course the SACD connections are through the six analog cables but the original question was only in reference to the signal coming out of the DirecTV box and into the AV receiver.

 

[j_garcia]

Plus the fact that I do not have PCM conversion turned on anywhere in the setup configuration continues to baffle me.

I think this is where you are confused - it doesn't matter if you have conversion turned on or not, a non "bitstream" signal (DD, DTS and variants), aka stereo, will be passed as PCM.

Like there is a option to use Dolby or you can select to convert Dolby to PCM, why would I want to do that?

You wouldn't because that would give you stereo only.

 

[MichaelJHuman]

For it to work the way I think you expected it to work, it would have to take the audio from the TV signal and encode it into Dolby Digital. They would require an onboard Doby Digital encoder! Which would be an unreasonable and costly feature.

So they chose the most direct and simple route, and they send PCM. It works exactly as it's supposed to if I understand your situation correctly.

 

[RickC3C4]

I think this is where you are confused -

J_Garcia,

You're right about my confusion, but I think I'm getting a handle on this, thanks.

 

[RickC3C4]

For it to work the way I think you expected it to work, it would have to take the audio from the TV signal and encode it into Dolby Digital. They would require an onboard Doby Digital encoder! Which would be an unreasonable and costly feature.

So they chose the most direct and simple route, and they send PCM. It works exactly as it's supposed to if I understand your situation correctly.

MichaelJHuman,

I do believe it is working exactly as its supposed to I just wasn't initially sure why it chose this path.

 

[MDS]

So we have a handle on PCM because j_garcia explained it well in his first post. There is no conversion going on inside the receiver.

- Some networks DO broadcast using PCM instead of analog or any flavor of DD. HGTV and Food Network on my cable system are always 48 kHz PCM. Some stations are always analog and some are always DD 2.0 or DD 5.1.

- DirectTV is satellite, which is always digital. IF the signal originated from the network as analog, DirectTV converted it to PCM before sending it over the airwaves. The satellite dish received it as PCM and the receiver is just processing it as PCM. If the receiver is in stereo mode it would show 2 channel PCM. If it is in a surround mode, it will pick the appropriate matrix decoder to turn it into 5.1 (PLII usually, although you can change it to whatever you want).