flac dev - Sep 2011 - mid-side coding and bits per sample

Dear list,

i'm doing a bit of analisys on flac's source code and i've run into
something i can't quite grasp.

flac version 1.2.1

flaclib C

stream_encoder.c

function "process_subframes_"

line 2999

++++++++++++++++++++++
    if(do_mid_side) {
        FLAC__ASSERT(encoder->protected_->channels == 2);
        for(channel = 0; channel < 2; channel++) {
            const unsigned w =
get_wasted_bits_(encoder->private_->integer_signal_mid_side[channel],
encoder->protected_->blocksize);
           
encoder->private_->subframe_workspace_mid_side[channel][0].wasted_bits =
encoder->private_->subframe_workspace_mid_side[channel][1].wasted_bits =
w;
            encoder->private_->subframe_bps_mid_side[channel] =
encoder->protected_->bits_per_sample - w + (channel==0? 0:1);
        }
    }
++++++++++++++++++++++++

In that piece of code the encoder determines how many wasted bits there are in
each channel (mid-side) and calculates effective BPS for each of them. What i
don't understand is why it should add one bit to the BPS of the side channel
"(channel==0? 0:1)", eventhough there is no "correction" to
wasted bits. Could someone shed light on this?
Thank you very much in advance!
 		 	   		  
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the side channel bps is 1 larger than the raw bps because it's a difference
between left and right channel values.



>________________________________
> From: Eri Eri <guguieri at hotmail.com>
>To: flac-dev at xiph.org 
>Sent: Monday, September 26, 2011 1:17 AM
>Subject: [Flac-dev] mid-side coding and bits per sample
> 
>
> 
>Dear list,
>
>i'm doing a bit of analisys on flac's source code and i've run
into something i can't quite grasp.
>
>flac version 1.2.1
>
>flaclib C
>
>stream_encoder.c
>
>function "process_subframes_"
>
>line 2999
>
>++++++++++++++++++++++
>??? if(do_mid_side) {
>??? ??? FLAC__ASSERT(encoder->protected_->channels == 2);
>??? ??? for(channel = 0; channel < 2; channel++) {
>??? ??? ??? const unsigned w =
get_wasted_bits_(encoder->private_->integer_signal_mid_side[channel],
encoder->protected_->blocksize);
>??? ??? ???
encoder->private_->subframe_workspace_mid_side[channel][0].wasted_bits =
encoder->private_->subframe_workspace_mid_side[channel][1].wasted_bits =
w;
>??? ??? ??? encoder->private_->subframe_bps_mid_side[channel] =
encoder->protected_->bits_per_sample - w + (channel==0? 0:1);
>??? ??? }
>??? }
>++++++++++++++++++++++++
>
>In that piece of code the encoder determines how many wasted bits there are
in each channel (mid-side) and calculates effective BPS for each of them. What i
don't understand is why it should add one bit to the BPS of the side channel
"(channel==0? 0:1)", eventhough there is no "correction" to
wasted bits. Could someone shed light on this?
>Thank you very much in advance!
>
>_______________________________________________
>Flac-dev mailing list
>Flac-dev at xiph.org
>http://lists.xiph.org/mailman/listinfo/flac-dev
>
>
>
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