[Alsaplayer-devel] bug in save/restore of the pause functionality

[Christoph Delfs]

Hi all,
I am wondering whether the problem Eric Winn has described earlier (in 2005) has been analyzed further in the past. I would like to cite from the mailing list below. Any comments from the audience?
Cheers
Christoph

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Hi Erik,

I will have a look at the problem. Sounds like a bug in the save/restore
of the pause functionality...

Thanks,
Andy

On Wed, Nov 05, 2003 at 11:18:27PM -0800, Erik Winn wrote:
> Hi,
> 
>  Tried a sort of brief question about this a few days ago - but it seems not 
> to have even made it to the list; I would very much appreciate some sort of 
> acknowledgement of this post if it is going to the private list (to which I 
> have subscribed but also without response ...) so that I know if I am talking 
> to the wall or not.
> 
>  For everything else, the latest version running with jackd 0.8 is great and 
> quite impressive (I especially like the reverse playback feature - neat 
> trick.), but I have found a nice glitch in pause/unpause funtionality that 
> causes incremental decrease in playback sample rates. 
> 
>  Here is the basic problem - I have isolated it to be related to the mad 
> plugin's interaction with CorePlayer::GetSpeed/SetSpeed after Pause when the 
> file being played is of a lower sample rate. For instance, I play an mp3 with  
> 56 kbps, 22 kHz (joint stereo), no CRC, no padding. At 44100, ap_get_speed() 
> returns 50 - which is correct scaling, but it seems that this is stored 
> somewhere and used to SetSpeed after pause. Here is what I have found so far:
> 
>  First, it saves a pitch point in CorePlayer::Start like this
> CorePlayer.cpp, line 585:
> 	output_rate = node->SamplingRate();
> 	input_rate = plugin->sample_rate(the_object);
> 
> 	if (input_rate == output_rate)
> 		SetSpeedMulti(1.0);
> 	else
> 		SetSpeedMulti((float) input_rate / (float) output_rate);
> 	update_pitch();
> 
>  ... well, mad_sample_rate will do this:
> 
> mad_engine.c, line 745:
> 	if (data)
> 		return data->samplerate;
>  
>  ... which, at this point should be 22050. Meaning, we call, effectively,
> SetSpeedMulti(0.5), after which pitch_multi gets 0.5 - and then update_pitch 
> essentially divides pitch in half:
> 
> CorePlayer.cpp, line 953:
> 		pitch = pitch_point;
> 	}
> 	pitch *= pitch_multi;
> 
>  ... Now, the next call to GetSpeed will return 0.5, so if we now pause and 
> unpause ... well, here it is:
> 
> void CorePlayer::Pause()
> {
> 	save_speed = GetSpeed ();
> 	SetSpeed (0.0);
> }
> 
> 
> void CorePlayer::UnPause()
> {
> 	if (save_speed)
> 	    SetSpeed (save_speed);
> 	else
> 	    SetSpeed (1.0);
> }
> 
>  The result is that everytime one pauses/unpauses, the speed is cut in half. 
> The same applies for 44100 samplerate mp3s if we are running jackd at 48000 
> and neglect to use -F 48000, frequency will be correctly scaled and 
> ap_get_speed will return 92, so every un/pause will reduce playback by 
> (previous_rate * 0.91875), and so on.
> 
>  I am quite unsure where to introduce a fix for this - I have only just 
> started looking at the code and spent but a few hours with it. I would be 
> happy to implement it and submit a patch if you would be good enough to 
> suggest where/how this might be done to fit and not break frequency scaling.
> 
> Best, and thanks for a nice player,
> Erik Winn
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