what is dual channel audio?

[doen1000]

Hi guys, I'm downloading a vcd movie with these specs :

Product Information
Format: 2 (VCD)
Language: CANTONESE, MANDARIN (Dual Channel)
Release Date: 9/13/2002
Subtitle(s): TRADITIONAL CHINESE SUBTITLES, ENGLISH SUBTITLES

What does the dual channel bit mean?

[mats.hogberg]

Sounds like "Stereo" to my ears...

/Mats

[epo]

Not stereo . It means that instead of left and right channel this video has (mono) audio channels in 2 differend languages.

[MOVIEGEEK]

When a s/vcd is recorded in dual channel it has two different languages/dialects.If you play with your DVD remote control audio button
while watching a s/vcd movie you will see what I mean.

[Mirror_Image]

What is "Joint-Stereo" ?

"Joint-Stereo" is a special stereo encoding mode that provides a better sound quality with low bitrates by mixing some frequency bands in mono, but only when necessary.

To understand what happens in this mode, one should know that :
- An MPEG file is composed of 'frames'.
- The frame size depends on the bitrate.
- Each frame is meant to contains all encoded sub-bands (at time 't').

The encoder allocates bits in a frame step by step, until there is no more room. Sub-bands that are close to the noise threshold are considered first, so that they gain more bits than other sub-bands. This leads to a bad quality if the frame size is not large enough, because sub-bands will not get enough bits.

In joint-stereo mode, the encoder first try to figure out whether all sub-bands will fit into the current frame in pure stereo. If not, it mixes the upper sub-bands in mono to lower bits requirements then check whether it will suffice. If not, it mixes more sub-bands, and so on until all sub-bands are mixed.

The bits allocation works as usual, but as mono sub-bands will need less bits than stereo ones, the encoder will have more room to put these sub-bands into the frame.

Thus, joint-stereo allow a better quality for a given bitrate, but to the detriment of stereo.

[doen1000]

thanks for the replies.

@ Mirror_Image

Is dual channel audio the same as joint stereo

[Mirror_Image]

Is dual channel audio the same as joint stereo?

Ah, sorry, I didn't put that in there did I (I did plan to, but I didn't). Dual chennel is not the same as joint stereo. Dual channel is pretty much 2 mono channels, not any sort of stereo signal.

[vitualis]

Dual-channel, standard stereo and joint stereo are all MPEG audio encoding modes of STEREO (in so far as the stuff in the left channel can be different from the stuff in the right channel).

Joint stereo as explained before. It exploits redundancy (i.e., similarity) between the left and right channel when encoding. This generally yields better quality at lower bitrates but there is the cost that there is a loss of stereo separation. This can have a large impact on Dolby Surround encoded audio.

Standard stereo does not exploit the similarity between left and right when encoding so it is on one hand less efficient, but on the other hand, you don't risk affecting the stereo separation. It can allocate a higher percentage of the total bitrate to the channel that has more "need", however (i.e., if L is silent and R is not, the majority of the bitrate can go to the R channel). At the higher bitrates commonly used for VCD (i.e., 224 kbit/s MP2), there will be little difference between the quality of standard stereo and joint stereo. I usually recommend that people use standard stereo for everything.

Dual-channel is the encoding mode where exactly half the bitrate is given the L and half to the R for the entire audio. It is used on some dual mono soundtrack encodings, but I question the logic behind that. IMHO, standard stereo does not offer any disadvantages over dual-channel in such a setting. Indeed, even J-S may be a better choice as even in dual languange soundtracks there will be a high degree of similarity between left and right and you wouldn't be worried about loss of stereo separation.

Regards.