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I have various simple short sound effects (damage sound, dying sound, thunderbolt, fanfare, breaking) for a game that is developed for Android currently.

I use OGG files: 96kbps VBR, 44.1KHz, 2 channels (that means stereo, right?).

I read the other stackexchange topics about "acceptable sound quality", but they're too general, address too many things. My experience is that even with 80kbps, my effects sound OK. But I tested it on a limited number of Android devices (including a Sony Ericsson Xperia Neo and a HTC Desire HD). My questions:

  1. For mobile phones and tablets, generally, what parameters are recommended? Won't my 80kbps sounds be bad on a newer device (such as a modern tablet)?

  2. I don't hear any difference between stereo and mono (2 channels vs. 1 channel, right?), is there any noticeable difference at all for mobile phones / tablets? (in terms of the player experience) May it worth it at all? I assume that stereo sounds take much more in memory (when they're decoded to PCM), despite of the fact that the compressed OGG size is practically the same.


Reacting to Roy T.'s great comment:

  1. Actually, I couldn't measure the PCM size (Android decodes OGG internally), but I thought that stereo will take more space than mono when uncompressed

  2. After throwing out one of the WAV channels in Audacity, and re-exporting it:

    • The new WAV file size is half than before
    • The OGG file size is practically the same as before
  3. The sound effects and game music was recorded by my friend who is an experienced hobby musician/composer, but he knows little about computers & software so he just gave me some high-quality WAV files generated via his hardware.These were stereo, but if I check them in Audacity, both channels appear to be exactly the same.Can I consider them the same (=> moving to mono), or might there be some unnoticeable differences to the human eye?

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Are your sounds affects actually stereo? If the compressed OGG size is practically the same but the PCM size isn't I would assume that the left and right channel don't have different data. In that case go for mono, there will be no quality penalty. –  Roy T. Jul 1 '12 at 16:33
    
Very very good point, Roy! I'll specify more details in the original post, and I'd appreciate if you could check them. (I can't put them here because they need nicer formatting.) –  Thomas Calc Jul 1 '12 at 16:42

2 Answers 2

up vote 2 down vote accepted

It's a balancing act. If you're trying to save memory, package size or processing time, lower quality mono sound is better. And of course, the acceptable quality is entirely up to you to decide if it's acceptable enough.

However, I believe you might be missing some use cases. Keep in mind that people can attach their mobiles and tablets to:

  • Headphones (very common for public places)
  • External powered speakers
  • Even their home entertainment system via HDMI.

So there is a reason for strive for higher quality sound than can be appreciated through the tiny speakers on many mobile devices. Stereo sound would be important for any listed situations and the lower on the list you go the higher quality the sound needs to be for optimal appreciation.

To address #3:

You can check to see if they are the same. Split the channels, invert one and subtract it from the other. If there's nothing left afterward, then you know they were the same. See here for steps on how to do that.

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Thanks, this is a very good point! As far as we've noticed, some mobile devices have a poor headset outlet (such as HTC Desire), as they have some noisy sound in it, which doesn't seem to be related with our game music. Somewhere on the internet we read that not all manufacturers have good outlets or headset sound quality. But probably these devices are getting even better and better, so your point is very current and real. What do you think, if I need to choose, which is more preferable: higher kbps or stereo vs. mono? If I want to keep a higher quality. –  Thomas Calc Jul 1 '12 at 16:50
    
On a side note, actually, if both channels of my sounds are the same (as Roy T. mentioned), then keeping the stereo doesn't make any sense, right? In this case, the way I can keep a higher sound quality is choosing higher kbps when exporting to OGG, if I understand correctly. –  Thomas Calc Jul 1 '12 at 16:51
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Correct. Keeping the sound files mono, but outputting them to stereo is a great way to keep sound files small, but high quality. It removes duplicate data. –  Byte56 Jul 1 '12 at 17:37
    
Great. The last thing I need to know is point #3 in my (extended) original post: do I need to prove via analyser software (?) that both channels are the same in my music files, or is it enough that they look totally the same in Audacity when I watch them? (I'll ask my composer friend too about what music he made, but as I said, he is more of a musician than someone familiar with digital music.) I mean, I want to be sure that I don't miss anything by throwing out the second channel. –  Thomas Calc Jul 1 '12 at 17:44
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@ThomasCalc why not just listen to both versions on your PC. If you can't hear the difference on a PC with proper speakers than you can't hear it on a tablet. –  Roy T. Jul 1 '12 at 19:36

If it is background music, then I suggest keep it stereo.

If they are sound effects (like jumping, shooting, etc) then keep them mono. It doesn't make sense keeping shooting sound stereo. Instead mono sound should be mixed to stereo depending where on screen does your character currently is located. For example, if it is at right screen edge - then the shooting sound should come from right speaker.

Also - I would say 44.1 kHz is unnecessary for game sounds. Keep them 22.5. It will reduce size twice. And will be 2x as fast for audio mixing (this depends on what library/API you are using for sound mixing/playback),

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"If it is background music, then I suggest keep it stereo." -- even if both channels contain the same sound data? Which looks like so in the case of my music files. About choosing the speaker: I don't think I can control which speaker to use -- the game is for tablets and mobile phones. Of course, as Byte56 pointed out, it might be attached to a headset or other output, but since mobile and tablet screens are not big, I'm not sure it would make any difference to choose speaker based on character position. –  Thomas Calc Jul 1 '12 at 17:13
    
What happens for mono sounds in case of speakers (e.g. two speakers)? Both speakers emit the same sound, right? Instead of one of them being silent while the other one is emitting the mono sound. (Sorry for the silly question, but I want to be 100% sure.) –  Thomas Calc Jul 1 '12 at 17:14
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Well it depends on audio library/API. Usually by default everybody will duplicate mono channel to both stereo channels exactly same. If your music contains same audio data in both stereo channels then of course keep the audio file mono. Why do you think you can not control? It doesn't matter what user uses, you use your audio library like it was always stereo. If the device is has only one speaker, then audio library will take care of that for correctly playing back stereo sounds. –  Mārtiņš Možeiko Jul 1 '12 at 17:18

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