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Topic: New DSP ATSurround matrix decoder (Read 20805 times) previous topic - next topic
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New DSP ATSurround matrix decoder

Reply #50 –
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But the problem is that when I have a file without surround information CMSS upmixes it to 5.1, while ATSurround outputs only FR,C,FL (plus bass redirection) and, as I've said, prohibits CMSS from upmixing it further. Which leaves rear speakers idle.

Suppose an audio file has been encoded with surround data. But for most of the audio, only the front channels are used, the creator only uses the surround channel part of the time for special effects. It was by design that the rear speakers are idle in this case. Unless the file had a flag stating whether it is to be decoded in surround or not, it would be impossible to determine the mode a decoder should operate. ie, should the decoder use the rear channels even when no surround data is encoded, or is no surround data present because the author of the sound chose not to encode it? As a surround decoder, ATSurround has to assume the latter case. (Note: most stereo audio streams do not have a surround data bit)

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However, other software DPL decoders (like the one in InterVideo WinDVD) can indicate when input is DPL-encoded. Which means that somehow they can tell the difference.

Dolby Digital (AC3) audio streams, like those on DVDs, when encoded to 2 channels, may contain a flag to inform the decoder (in this case the decoder in WinDVD) that the sound should be Dolby Pro Logic decoded. Which could possibly explain why WinDVD is able to feedback that the input is DPL-encoded. Or, it could also reflect DPL decoding even for common formats like MP3 or WAV, but this does not mean that the audio has been surround encoded.

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Also, can't mute surround (rear) channels tell ATSurround that the stream probably has no surround information at all? I bet most music files either have encoded rear channel signal right from the start or altogether have it not (for example, when a surround-encoded Audio CD gets reencoded to a lossy format without a DPL-aware medium or using lossy stereo mode that ruins positioning information).

In most music today (recorded onto stereo mediums, eg. CDs), there aren't really such things as mute surround channels. This is because effects like reverb, choruses and other forms of filters do affect the phase of the audio somewhat, and results in out-of-phase information, which is interpreted as surround. Therefore, some surround data is inherently present, I couldn't use the method of detecting mute surround channels to disable the rear channels.

As far as I know, Audio CDs do not contain information on whether the audio has been encoded with surround or not. (With the exception of DTS CDs and SACDs, which may contain multichannel audio, but cannot be played back on normal CD players. HDCDs with encoded digital data also exist, but contain no surround data flag, they can be played on a normal player.)

So this problem is a tough one. However, I'll keep mulling it over in my head. Thanks for your comments
Andrew Tan

New DSP ATSurround matrix decoder

Reply #51 –
Thank you very much for your explaining these things to me!

New DSP ATSurround matrix decoder

Reply #52 –
sorry if this is naif, but are there similarities between ATSurround and Dolby Headphone?
Could I use ATSurround in order to encode my 5.1 music -> 2CH stereo -> MP3 for my iPod?
Would I get something similar to Dolby Headphone?

I know I could just try by myself, but I'm away from my hardware for a couple of weeks...

thank you

ciao -- Nando

New DSP ATSurround matrix decoder

Reply #53 –
atjs, is it possible to have ATSurround version for Winamp? No, I do not use Winamp, but my video player (BSPlayer) can use Winamp plugins, and many would appreciate a free software DPL-decoder for viewing movies with DPL-encoded sound. Am I right assuming that a DSP plugin could function that way?

New DSP ATSurround matrix decoder

Reply #54 –
Quote
sorry if this is naif, but are there similarities between ATSurround and Dolby Headphone?
Could I use ATSurround in order to encode my 5.1 music -> 2CH stereo -> MP3 for my iPod?
Would I get something similar to Dolby Headphone?
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The similiarity between them is that they can convert multichannel sources (eg. 5.1) to stereo.

However, the difference is that ATSurround Encoder performs an operation called matrix encoding to embed the surround into the stereo audio (no audible change results using headphones), while Dolby Headphone uses head-related transfer functions (HRTFs) to process the sound to make it appear as though it were coming from a large theatre (to remove the "in-head" feeling of the music).

The ATSurround encoding process is reversible, allowing you to reproduce the multichannel recording using the ATSurround Decoder or your consumer equipment. However, if you are going to play music on your portable player on headphones, Dolby Headphone might be a better choice.

You can convert an audio file using ATSurround, by placing ATSurround Encoder into the DSP stack (remember to remove ATSurround Decoder), loading a multichannel file into the foobar playlist, right-clicking on the file and selecting "Convert".

My suggestion is to use ATSurround if you're going to play it on a multichannel speaker system and Dolby Headphone if you're going to play it on headphones. 

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atjs, is it possible to have ATSurround version for Winamp? No, I do not use Winamp, but my video player (BSPlayer) can use Winamp plugins, and many would appreciate a free software DPL-decoder for viewing movies with DPL-encoded sound. Am I right assuming that a DSP plugin could function that way?
[a href="index.php?act=findpost&pid=349733"][{POST_SNAPBACK}][/a]

You're in luck, I released the Winamp version of the decoder on the 10th of December. However, do be warned that it is still a very early release and may contain bugs. Also, it is an output plugin and not a DSP plugin. The lack of bit resolution depths above 16 bits is not yet supported, but I am working on it. Meanwhile, you can get the latest version on my website at [a href="http://www.andrewlabs.com]http://www.andrewlabs.com[/url]

Edit: So sorry, saw that BSPlayer only supports DSP plugins and not output ones. In that case then my plugin would not work. Hopefully, the authors of BSPlayer might include support for Winamp outs. Link to BSPlayer topic

Sorry for the delay in replying. Blessed Christmas! 
Andrew Tan