[FFmpeg-devel] [RFC] LPCM 24 bits support

[Giuliano Pochini]

On Sun, 13 Apr 2008 09:18:16 +0200
Lars T?uber <lars.taeuber at gmx.net> wrote:

> Hallo Giuliano,
> 
> I overlooked your work sorry for that.
> 
> On Sun, 30 Mar 2008 18:56:17 +0000 Giuliano Pochini <pochini at shiny.it> wrote:
> > diff -dupr ffmpeg__orig/libavcodec/pcm.c ffmpeg/libavcodec/pcm.c
> > --- ffmpeg__orig/libavcodec/pcm.c	2008-02-06 12:37:37.000000000 +0000
> > +++ ffmpeg/libavcodec/pcm.c	2008-03-30 18:41:30.000000000 +0000
> > @@ -492,6 +492,43 @@ static int pcm_decode_frame(AVCodecConte
> >              *samples++ = s->table[*src++];
> >          }
> >          break;
> > +    case CODEC_ID_PCM_DVD20:
> > +    {
> > +        int audio24[8*2], *ap;
> > +        const uint8_t *src_LSB;
> > +
> > +        for (n=buf_size/(avctx->channels * 5); n>0; n--) {
> > +            ap = audio24;
> > +            src_LSB = src + avctx->channels * 2 * 2;
> > +            for (c=0; c<avctx->channels; c++, src+=4, src_LSB++) {
> > +                *ap++ = (((const int8_t *)src)[0]<<16) + (src[1]<<8) + (*src_LSB & 0xf0);
> > +                *ap++ = (((const int8_t *)src)[2]<<16) + (src[3]<<8) + ((*src_LSB << 4) & 0xff);
> 
> Does this work with an odd number of channels, or are only even number of channels possible?

Yes, it does. I tested it with 1, 2 and 6 -channels files. It works because
each block contains two frames. A 24-bits mono stream is encoded as follows:

0:	hi byte channel 0 frame 0
1:	mi byte channel 0 frame 0
2:	hi byte channel 0 frame 1
3:	mi byte channel 0 frame 1
4:	lo byte channel 0 frame 0
5:	lo byte channel 0 frame 1

A 20-bits mono stream is the same, except byte #4 contains two nibbles.


> > +        }
> > +        break;
> > +    }
>
> I'm not sure, but the break; and the following { should be exchanged.!?

I copied the coding style of adpcm.c, where there are several unconditional
code block inside case's where the break is inside the closing brace.


> > +    case CODEC_ID_PCM_DVD24:
> > +    { [...]
> > +        break;
> > +    }
>
> break; <=> }    ??

Same as above. Is there a coding style doc ? I could not find any.


--
Giuliano.