Technical Question Regarding Sound Formats

DJG

Standard Member
Hi

What is the difference between various sound encodings like Dolby and Linear PCM.

I understand that surround sound is encoded on a DVD or BD in formats such as Dolby or DTS. And I think I am right in thinking that Linear PCM is uncompressed Audio.

So let’s say I have a BD with Dolby encoding. The PS 3 sends this to my AV receiver (A Sony STR-DA2400ES). The AV receiver then decodes the signal and sends it to the appropriate speaker.

However, on the screen on the AV receiver, it always says Linear PCM, no matter what the encoding on the source is (it says Linear PCM whether it is a BD / DVD with Dolby / DTS, a CD sending stereo, or just the TV sending (presumably) stereo.

What’s going on? What is the information flow sequence? Does the AV receiver just convert everything to Linear PCM irrespective of the original format?

Thanks

Dave
 

lechacal

Novice Member
Yes LPCM is uncompressed audio.

If the amp is receiving a Dolby/DTS bitstream then it will say so. If the amp is telling you that you have LPCM then your PS3 is decoding the audio from the disc and sending it all as LPCM to the receiver.

I don't know about the PS3 but most DVD/BD players have the ability to decode to LPCM or pass the proprietary audio stream to something else via a digital output, depending on how you have them configured. There's pros and cons to either approach and people debate them but there's very little (if anything) in it.
 

DJG

Standard Member
Thanks for the reply.

That makes more sense now with regards to the PS3 deecoding DVD and BD.

So, when the PS3 decodes CD audio, does it send that as Linear PCM too?

When the receiver recieves audio from the TV, that says linear PCM too. How can the TV be sending this?

Thanks again

Dave
 

lechacal

Novice Member
Audio is stored on a standard CD in LPCM format.

Your TV is receiving the compressed audio in the incoming stream. If you have a digital connection between the TV and amp then you may find that DD audio gets bitstreamed to the amp (depending on a setting on the TV). Anything else will get decoded to LPCM by the TV.
 

DJG

Standard Member
Ahh, I see.

So whatever the receiever screen displays, in this case "Linear PCM", is this what is ultimatley begin sent to the speakers?

So for example, the receiver gets a signal from a DVD / BD via the PS3, be it Bitstream then decoded or linear PCM pre decoded, then the reciever is telling me it's sending the signal to the speakers as linear PCM?

If that is correct, is it theoretically possible for the receiver to send the signal to the speakers as some other format. For exampe, 10 years ago we had surround sound, but the receivers could not deal with linear PCM. If you but a Dolby signal into the receiver, and it got decoded, how was it sent to the speakers?

Thaks again

Dave
 

lechacal

Novice Member
No, not quite.

Ultimately what gets sent to the speakers is an analogue electrical signal. Until relatively recently all inputs to an amp would have been weaker (line-level) anaolgue signals that got, well...amplified.

LPCM is an uncompressed digital audio format. It is what is stored on CDs and is what uncompressed audio is generally passed around as over digital links (optical, coax, HDMI). Amps these days can deal with digtal inputs as well as analogue and an LPCM signal just gets converted to an analogue one and amplified.

More recent amps are also able to decode incoming compressed formats (DD, DTS and the newer HD ones for example). These are decoded to LPCM, converted to analogue and amplified. If the amp is receiving one of these formats it will usually say so on the display. If it does then you know the amp is decoding the audio. If the amp says it's receiving LPCM then something else has decoded the signal before it gets to the amp (if it was necessary - it isn't for a CD for instance).

In your last paragraph where you talk about pre-LPCM surround I think your referring to Dolby Surround/Dolby Prologic. These were formats stored in an analogue, 2 channel signal which was then 'de-matrixed' by the amp (using some analogue electronics) to produce more than 2 channels of analogue sound and then amplified).

There are speakers that have internal amps (active speakers) but I didn't know of any stand-alone speakers with digital inputs that can do their own decoding and amplification. The digital signal would have to be split and the speaker would need to know which position it's in. I suppose it's possible but I don't know why you would do it. Speakers (as far as I know) always take an analogue signal which is either at line-level (active speakers) or an amplified signal.
 

DJG

Standard Member
Thanks for the superb reply, this all makes far more sense to me now.

Just out of interest, in the scenario that my PS3 is doing the decoding (which after your reply I’m pretty sure it is as the AV receiver says Linear PCM on it when receiving a PS3 signal), if the signal has already been decoded to linear PCM, how is the surround information passed on.

For example, the PS3 is sending a linear PCM signal, which the AV receiver detects and deals with, yet the AC receiver multi channel lamp comes on. Therefore, the AV receiver "knows" it is a multichannel signal. If at this point it is already a linear PCM signal, how does the information regarding what speaker to send what sound to get through. I guess what I’m asking is, in the case of a pre decoded surround signal, how is the surround sound distribution information passed on?
 

lechacal

Novice Member
When I say they are 'decoded to LPCM', I mean they are decoded to a number of channels of LPCM, depending on how many channels there are in the encoded audio stream. The amp will be receiving several discrete LPCM streams from your PS3 via the HDMI cable.

A normal audio CD actually contains two separate LPCM streams for two channels of audio (left and right).
 

safcalibur

Distinguished Member
Hi mate,

If your PS3 is doing the decoding (is set to LPCM) then the display on your amp would read something like 'Multi Channel In'. The sound is sent to the correct speakers but as your amp doesnt know the origin of the audio it displays 'multi channel in'. Whether the sound is compressed or not wont affect how the sound is channeled to each speaker. It just depends on how many channels of sound have been encoded e.g. 2 (stereo) 5 or 7 (surround). If your PS3 is set to bitstream then when your amp recieves the signal it will decode it and thats when your amp will show what sound format it is e.g. DD, DTS etc...

cheers

Saf
 
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