some thoughts about the (removed) clipping prevention

[PaulJam]

In the last days there was a lot of discussion (on irc and the developer-mailinglist) about the nwe feature "prevent clipping" which was removed today. Here is my opinion about this feature:

I use the bass bosst with a value of +18 db, so the music might clip if i set the volume above -18 db and i don't want my music to clip on the player. This means every time i use a higher volume i need to go to the soundsettings menu and lower the bass/treble that volume + bass-/trebleboost doesn't go over 0 db (which would otherwise mean possible clipping). There are situations where i don't want to take my player out of the pocket every time i change the volume (for example when riding my bicicle). In my opinion the basic functions of a portable player should be savely operateable inside the pocket without having to look at the display.
If you like the music loud and don't mind clipping you could always disable the option.
I don't see this option as "protecting the user from himself", i see it as "automating things the user would have to do manually".

Another useful side-effect of this feature would be that if you connect your player to the home stereo you simply have to set the volume to 0 db and you have a flat equalizer, which is most times desired when using the line out.

I don't like this feature because the original firmware behaves like this, i like it because for me it is a useful feature. I hope this Feature gets re-implemented in RockBox, but maybe with less options (on/off would be sufficient imho).

Links:
developer-mailinglist(Simplified and uniform volume handling - asking for opinions)

What are your opinions on this issue?

PS: I hope this is not too hard to read - english is not my native language.

[belly917]

I just read through today's IRC logs and one of the devs mentioned some of the reasoning behind this change and what could "possibly" happen in the future.

First, the prevent clipping removal & -db volume scalling change was done to provide a more "pure" "un-fooled-around-with" signal from the player. And also to purge rockbox of the extrenious settings that controlled the previous settings. The developers want to keep rockbox unbloated.

As far as the industry norm, the Iriver firmware is the oddball in that it scales back the bass boost to prevent clipping. Try turning up the volume on a Sony Cd player with mega bass on.. it'll distort & clip like crazy. So rockbox isn't doing anything unusual here.

Originally Posted by PaulJam

I don't like this feature because the original firmware behaves like this, i like it because for me it is a useful feature. I hope this Feature gets re-implemented in RockBox, but maybe with less options

One of the devs mentioned that they may try to reintroduce clipping prevention again in the future, but they would only scale back the bass.. not the volume, and introduce this through leaner settings.

I think the origional clipping prevention of capping the volume was a poor solution, and I am glad to see it gone. I do/did enjoy the iRiver method of scaling back the bass, but I only needed this feature when in very loud environments. The volume doesn't get that high on a day to day basis. So, for the time being, the lack of clipping prevention doesn't bother me.

p.s.

Originally Posted by PaulJam

Another useful side-effect of this feature would be that if you connect your player to the home stereo you simply have to set the volume to 0 db and you have a flat equalizer, which is most times desired when using the line out.

Why not just make a cfg file or 2 that will handle these changes for you (and put them in the themes folder. Here's some examples:

# config for using analog stereo connection

wps: /.rockbox/WPS/simply-large.wps
font: /.rockbox/fonts/nimbus-12.fnt
spdif enable: off
statusbar: on
volume: 0
bass: 0
treble: 0

# config for using optical connection to stereo

wps: /.rockbox/WPS/simply-large.wps
font: /.rockbox/fonts/nimbus-12.fnt
spdif enable: on
statusbar: on
volume: -84
bass: 22
treble: 0

[PaulJam]

Why not just make a cfg file or 2 that will handle these changes for you (and put them in the themes folder. Here's some examples:

Thank you for the information. (i always thought a cfg-file has to contain all settings, which would have made it hard to maintain several cfg-files)

[MartinP]

I'm a bit of a purist when it comes to audio, so I don't listen with bass or treble boost. However, I'm not going to let that stop me from spouting about this issue.

The iRiver way to avoid clipping gives max volume, at the expense that people don't understand it. As someone else said, the Fletcher-Munson curves mean that less bass boost is required as volume is increased. This might actually imply that the bass boost should be gradually diminished as volume increases ie before the clipping limit is reached. Unfortunately, different headphones would need different response compensation, so this would be a real can-of-worms! It's also worth pointing out that the "loudness" control on some hifi's operates in this way - only applying a bass/treble boost when the volume is quiet.

The old RockBox way gives what the user actually asked for, at the expense that quiet/low bass tracks can't be played loud, even if they wouldn't actually clip.

I certainly agree that allowing the user to select between the two options is desirable.

With the RockBox method, could a further option be added that allows the volume to be increased beyond the "clipping" point, but only if the volume button is held down for five seconds?

I suspect that it would also be extremely easy to detect where clipping has taken place. Using normal 16 bit arithmetic, anytime the value exceeds the maximum values of +32,767 and -32,768 an "overflow" condition will be detected, and a "rubbish" value generated. Presumably, the bass/treble boost function has to monitor for that "overflow", and substitute either +32,767 or -32,768 for that "rubbish" value.

Could the WPS display a "clipped" indicator?

cheers, Martin


PS one point about all these different clipping options - how many headphones can actually cope with the maximum volume without clipping anyway?

[PaulJam]

The iRiver way to avoid clipping gives max volume, at the expense that people don't understand it.

With the new dB-volume it is imho very transparent how this feature would work: if the volume is -18dB, then the maximum bass-boost is +18dB. If the volume is -12dB, then the maximum bass-bost is +12dB.
I see no reason why people wouldn't understand this. It only has to be documented.

I suspect that it would also be extremely easy to detect where clipping has taken place. Using normal 16 bit arithmetic, anytime the value exceeds the maximum values of +32,767 and -32,768 an "overflow" condition will be detected, and a "rubbish" value generated. Presumably, the bass/treble boost function has to monitor for that "overflow", and substitute either +32,767 or -32,768 for that "rubbish" value.

Could the WPS display a "clipped" indicator?

As far as i understand clipping that results from too high bass-/trebleboost can't be detected because the bass-/trebleboost is done in the DAC and the audio can only be analyzed before it goes to the DAC.
Clipping that results from too loud mp3s or too much preamp can be detected in the wps (the Peakmeter has a clipping indicator), but i think i have read somewhere in the RockBox wiki that this is not 100% reliable (clipping is not always detected).

PS one point about all these different clipping options - how many headphones can actually cope with the maximum volume without clipping anyway?

This is not only about the maximum volume. The digital clipping can (theoretically) aleredy happen at a volume -23dB if the bassboost is set to 24dB. And i think most headphones can deal with volume -23dB.

[MartinP]

This is not only about the maximum volume. The digital clipping can (theoretically) aleredy happen at a volume -23dB if the bassboost is set to 24dB. And i think most headphones can deal with volume -23dB.

Surely, if the volume is -23 and the bass boost is +23 then the volume of the bass is zero, not -23.

I presume it's the bass that will cause clipping.

The new control layout sounds OK. Although negative dB volume controls are not really intuitive to the majority of people, at least it makes the link between bass & volume explicit.

I still think it might be desirable to add a pause to the volume when moving past the theoretical clipping point.

cheers, Martin

[MartinP]

Originally Posted by PaulJam

As far as i understand clipping that results from too high bass-/trebleboost can't be detected because the bass-/trebleboost is done in the DAC and the audio can only be analyzed before it goes to the DAC.

Stupid of me!! I knew that.

cheers, Martin

PS Will be applicable if RB ever write add a "graphic equalizer" function, which I've seen discussed somewhere. Not applicable to the current setup, though.