# Ac3 5.1 audio track convert to stereo

1. my research and conclusion is
1 channels = mono = 1 signal for all speakers
2 channels = streo = 2 signals(1st signal for left and 2nd signal for right) for all speakers

more channels is used to create sound effects like car is moving right to left
but also more channels makes file size bigger
2. Originally Posted by kirito
my research and conclusion is
1 channels = mono = 1 signal for all speakers
2 channels = streo = 2 signals(1st signal for left and 2nd signal for right) for all speakers
Where the sound goes depends on what speakers you have and how the amp is set up. Mono may only be sent to the front center speaker. Or to both the front left and front right speakers, or all three front speakers. Or to all speakers. Stereo may be sent to the front left and front right speakers. Or all three front speakers. Or, when left and right channels are identical, only to the front center speaker. When the left and right channels are exactly 180 degrees out of phase the sound will be sent to the rear left and rear right speakers, or the rear center speaker on systems so equipped.

Originally Posted by kirito
more channels is used to create sound effects like car is moving right to left
Left to right can be done with stereo. More channels add front/back placement and ambient effects (echo, reverb, etc.).

Originally Posted by kirito
but also more channels makes file size bigger
Not necessarily. With uncompressed or losslessly compressed audio more channels make for bigger files. But with bitrate based encoding (mp2, mp3, ac3, aac, etc.) the bitrate determines the size. 2 channels at 448 kbps is the same size as 6 channels at 448 kbps. Stereo mp3 at 384 kbps is the same size as stereo ac3 at 384 kbps.

The definition of bitrate is:
Code:
`bitrate =  size / time`
For audio size is usually expressed in kilobits and time in seconds, so you get kilobits per second, kbps.

So when you specify the bitrate you're actually specifying the size (since the running time is fixed). Simple algebraic manipulations gives:

Code:
`size = bitrate * time`
For example, 60 seconds at 128 kbps:

Code:
```size = bitrate * time
size = 128 kbps * 60 seconds
size = 128000 bits * 60 seconds
size = 7680000 bits
size = 960000 bytes (8 bits per byte)
size = 960 KiloBytes
or
size = 937.5 KibiBytes```
Of course, the full size of the file will be a little larger because there's a little container overhead.
3. Alternatively to plain stereo you can use sofa filter and produce audio more suitable for headphone listening, i personally consider this as subjectively better.
Normal people has two ears and stereo is minimum to provide acceptable audio field - if you desperate to save bitrate then you can use mono.

Bellow sofa downmixing filter chain for ffmpeg:
Code:
`-af "aformat=channel_layouts=7.1,aresample=resampler=soxr:osr=48000:cutoff=0.99:dither_method=0,sofalizer=sofa=dodeca_and_7channel_3DSL_HRTF.sofa:gain=11:lfegain=9"`

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