IPB

Welcome Guest ( Log In | Register )

 
Reply to this topicStart new topic
ADPCM vs PCM -> how lossy?
Emanuel
post Feb 27 2002, 21:52
Post #1





Group: Members
Posts: 284
Joined: 13-January 02
From: Sthlm, Sweden
Member No.: 999



In Creative Labs new USB based soundcard Extigy, they switch to ADPCM on the fly to achieve more channels than two to overcome the bandwith limit of USB. Otherwise, the limit would be just two channels.

My question is if anybody here knows how a 4:1 ADPCM audio coding affects the sound quality compared to ordinary PCM coding.

The technichal part of me says "wait until Creative releases a usb 2.0 version" to adress this issue, but if the sound quality for ADPCM is not an issue - it will probably only be needed for plaing back Audio DVDs and DVD movies - then why wait?

Peace,

Emanuel
Go to the top of the page
+Quote Post
rjamorim
post Feb 27 2002, 22:03
Post #2


Rarewares admin


Group: Members
Posts: 7515
Joined: 30-September 01
From: Brazil
Member No.: 81



You can test the effects of 4:1 ADPCM encoding yourself. All windows 9X version come with IMA ADPCM and M$ ADPCM installed as ACM codecs. Use an editing program to convert a wave file to ADPCM (Use IMA, it's slightly better than Microsoft's) and listen to both to see if you find any sound degradation.

Of course, there will probably be differences between Creative's ADPCM and IMA's. But that test already gives an idea of the pros and cons.

Concluding: Of course, there will be sound degradation. But you won't find there artifacts common to psychoacoustic encoders, like pre-echo.

Regards;

Roberto.


--------------------
Get up-to-date binaries of Lame, AAC, Vorbis and much more at RareWares:
http://www.rarewares.org
Go to the top of the page
+Quote Post
Emanuel
post Feb 27 2002, 22:12
Post #3





Group: Members
Posts: 284
Joined: 13-January 02
From: Sthlm, Sweden
Member No.: 999



Thanks for your answer, Roberto. I have already tested the method, but for now only on a relatively cheap audio system - and it is very difficult to judge where the artifacts are. Until I upgrade the system and can judge for myself, I wonder what kind of known/common problems there are with ADPCM audio coding. If pre-echo is not an issue, does it cut in the frequency range?

Peace,

Emanuel
Go to the top of the page
+Quote Post
Trelane
post Feb 27 2002, 22:20
Post #4


Crossfeed Developer


Group: Developer
Posts: 253
Joined: 24-January 02
From: Enola, PA, USA
Member No.: 1152



The only thing you'll probably notice is a slight increase in noise. This is because ADPCM uses prediction and a predefined index of sample values. These predefined are used instead of the real value.

So if you have, say, 283, 999, 391 as the input, ADPCM codecs may make them 290, 1000, 400.

It sounds pretty good, but it's not perfect wink.gif
Go to the top of the page
+Quote Post
tangent
post Feb 28 2002, 06:01
Post #5





Group: Members
Posts: 674
Joined: 29-September 01
Member No.: 63



You won't get pre-echos, dropouts, etc, but mostly quantisation noise with ADPCM. How ADPCM works is that it looks at the last few samples and predicts the value of the next sample. The prediction error is then scaled (adapted to the average volume of the source), quantised and encoded. Since 4:1 is used, that's 4 bits per sample for a 16bit source, so the error can only take 16 values per sample, so that's where the quantisation noise comes in. How well this encoder performs depends on a lot of things, including the complexity of the source (higher complexity = worse predictability), the source volume, and other things.
Go to the top of the page
+Quote Post
Emanuel
post Feb 28 2002, 11:48
Post #6





Group: Members
Posts: 284
Joined: 13-January 02
From: Sthlm, Sweden
Member No.: 999



Thanks everyone for clearing this out for me.

To whom it may interest: The times when this might be a problem may be limited afaik, since Extigy (when properly configured) handles the direct audiostream from lets say a DVD video with its internal DTS decoder. In that case it will not need to convert the audio channels to ADPCM to fit in the USB 1.1 interface.

Peace,

Emanuel
Go to the top of the page
+Quote Post

Reply to this topicStart new topic
1 User(s) are reading this topic (1 Guests and 0 Anonymous Users)
0 Members:

 



RSS Lo-Fi Version Time is now: 23rd July 2014 - 09:07