AC3 compressed domain transcoding possible?

[LigH]

Many people during the last years (or decades?) already wondered if one could reduce the bitrate of Dolby Digital audio without reducing the quality of the AC3 metadata due to complete decoding and re-encoding. Similar to DVD Shrink (MPEG2 Rejig), would a kind of "AC3 Shrink" be possible at all? Or is the relation between data and metadata too tight to make it happen?

I fear "If it were easy it were already done"...

[Blue_MiSfit]

I don't know about compressed domain transcoding, but frequency domain transcoding is certainly possible, at least when transcoding AC3 to E-AC3.

Dolby encoders can do precisely this - I'm most familiar with the Dolby DP600.

[LigH]

The idea behind CDT is (analog to DVD Shrink and Rejig) that the material is not decoded completely (to pixels, in case of video, and to samples, in case of audio), but processed in a format half-way into the encoding process (in case of MPEG2, only the quantization factors of the macroblocks are raised; I wonder if something similar is possible with AC3, e.g. removing only a few frequency parameters which already have very small coefficients — until the bitrate can be reduced e.g. from 640 to 448 kbps, or from 448 to 384).

[Blue_MiSfit]

Right. Sorry I should have been more clear. I'm well aware of the concept of compressed domain transcoding, but am not familiar with a real implementation of this in the AC3 world :)

Sorry!

[tebasuna51]

... e.g. removing only a few frequency parameters which already have very small coefficients — until the bitrate can be reduced e.g. from 640 to 448 kbps, or from 448 to 384).
Maybe is easy in MPEG2 with VBR frames, but AC3 must be CBR with a fix FrameLength along all the stream.

[Selur]

is there another way (using free tools) to reencode an ac3 stream to a lower bitrate and still keep drc and dialnorm?

[tebasuna51]

No problem with dialnorm but drc don't work fine with Aften.