I'm trying to create a piano-like app for iPhone, and currently recording every mp3 for a short tone for each key. Which sound quality better fits for that, 64kbps or 128 kbps?
That depends. Some people won't hear the difference, but if you're using headphones a lot of people will hear it, so use the higher quality (IMHO even 128kbps is way too low for MP3... I'd go for 192kbps).
As for CPU load: decoding and playing MP3 is so cheap on the iPhone that you won't notice any difference, the CPU load of a high-quality MP3 is "very low" instead of "ridiculously low". Don't worry about it (just for comparison, I'm doing realtime MPEG-2 video and audio decoding in software, even the iPhone 3GS can cope with that).
The lower the bitrate, the less processing required, and the lower the quality. Since your app is all about the music, it makes sence to have better quality. If the sound was just for background or effect, it might be acceptable to reduce the quality.
Related
I am using several instances of AVAudioPlayer to play overlapping sounds, and getting harsh distortion as a result. Here is my situation... I have an app with several piano keys. Upon touching a key, it plays a note. If I touch 6-7 keys in rapid succession, my app plays a 2 second .mp3 clip for each key. Since I am using separate audio streams, they sounds overlap (which they should), but the result is lots of distortion, pops, or buzzing sounds!
How can I make the overlapping audio crisp and clean? I recorded the piano sounds myself and they are very nice, clean, noise-free recordings, and I don't understand why the overlapping streams sound so bad. Even at low volume or through headphones, the quality is just very degraded.
Any suggestions are appreciated!
Couple of things:
Clipping
The "buzzing" you describe is almost assuredly clipping—the result of adding two or more waveforms together and the resulting, combined waveform having its peaks cut off—clipped—at unity.
When you're designing virtual synthesizers with polyphony, you have to take into consideration how many voices will likely play at once and provide headroom, typically by attenuating each voice.
In practice, you can achieve this with AVAudioPlayer by setting each instances volume property to 0.316 for 10 dB of headroom. (Enough for 8 simultaneous voices)
The obvious problem here that when the user plays a single voice, it may seem too quiet—you'll want to experiment with various headroom values and typical user behavior and adjust to taste (it's also signal-dependent. Your piano samples may clip more/less easily than other waveforms depending on their recorded amplitude.)
Depending on your app's intended user, you might consider making this headroom parameter available to them.
Discontinuities/Performance
The pops and clicks you're hearing may not be a result of clipping, but rather a side effect of the fact you're using mp3 as your audio file format. This is a Bad Idea™. iOS devices only have one hardware stereo mp3 decoder, so as soon as you spin up a second, third, etc. voice, iOS has to decode the mp3 audio data on the cpu. Depending on the device, you can only decode a couple audio streams this way before suffering from underflow discontinuities (cut that in half for stereo files, obviously)... the CPU simply can't decode enough samples for the output audio stream in time, so you hear nasty pops and clicks.
For sample playback, you want to use an LPCM audio encoding (like wav or aiff) or something extremely efficient to decode, like ima4. One strategy that I've used in every app I've shipped that has these types of audio samples is to ship samples in mp3 or aac format, but decode them once to an LPCM file in the app's sandbox the first time the app is launched. This way you get the benefit of a smaller app bundle and low CPU utilization/higher polyphony at runtime when decoding the samples. (With a small hit to the first-time user experience while the user waits for the samples to be decoded.)
My understanding is that AVAudioPlayer isn't meant to be used like that. In general, when combining lots of sounds into a single output like that, you want to open a single stream and mix the sounds yourself.
What you are encountering is clipping — it's occurring because the combined volumes of the sounds you're playing are exceeding the maximum possible volume. You need to decrease the volume of these sounds when there's more than one playing at a time.
Okay well i have alot of sounds on my app and just wondered what is the best audio format to use for the sounds?
Thanks
If your app will mainly focus on audio then you have two paths.
1) Your app wants to do pretty intensive audio processing (filtering, effects, etc). For this uncompressed audio is going to be your best bet. Using wav files or raw PCM data require no decompression and thus can be read much faster.
2) Your app wants to do less intensive audio processing. Simply mp3 or aac should be fine.
Also what will be your target devices? There is a big difference between what the iPhone 4 can do and what the iPhone 3g can do.
I know this is not a specific programming question but I hope someone can give me a suggestion. My applications (iPhone and Blackberry applications) use a lot of audio files. I need a solution for my applications in order to save some spaces.
Is it right that .aac is the most suitable audio format for iPhone? Is it the smallest one? It it also suitable for Blackberry?
Is there any way to make the audio files smaller without losing a lot of quality of the sounds? How about the bitrate, sampling freq and channels? Are they really matter?
AAC is a good format for the iPhone. The iOS is optimized to play AAC.
Yes, things like bitrate, sampling frequency and number of channels are all factors in the audio file's size.
What you should do is take your audio and convert it to different formats with different settings and then just play them on a real device to see if the quality is acceptable.
Sorry, there is no simple answer. Experiment.
Depends on what type of audio you're encoding. For speech, AMR is supported by all major smartphones, and will generally give the smallest file sizes. Quality degredation is noticeable enough that it's not suitable for music, but it's optimized for voice recording (the voice notes app on the BlackBerry uses it as its file format) so it'll give you very nice results with spoken audio.
What kind of audio files are you using in your iPhone games/apps?
I have a game with 30MB of sounds in .wav format and I'm thinking of maybe converting to .mp3 to reduce the app size... Is there a major difference in performance? Any other issues?
Keep in mind that certain codecs run in hardware and others in software. Therefore not all compressions will allow for simultaneous playback of more than one sound. For example, if you have a sound playing, a UI sound like a beep may not play if both were trying to use the same codec. For more info, see:
http://developer.apple.com/iphone/library/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/AudioandVideoTechnologies/AudioandVideoTechnologies.html#//apple_ref/doc/uid/TP40007072-CH19-SW6
iPhone Audio Hardware Codecs
iPhone OS applications can use a wide range of audio data formats. Starting in iPhone OS 3.0, most of these formats can use software-based encoding and decoding. You can simultaneously play multiple sounds in all formats, although for performance reasons you should consider which format is best in a given scenario. Hardware decoding generally entails less of a performance impact than software decoding.
The following iPhone OS audio formats can employ hardware decoding for playback:
AAC
ALAC (Apple Lossless)
MP3
The device can play only a single instance of one of these formats at a time through hardware. For example, if you are playing a stereo MP3 sound, a second simultaneous MP3 sound will use software decoding. Similarly, you cannot simultaneously play an AAC and an ALAC sound using hardware. If the iPod application is playing an AAC sound in the background, your application plays AAC, ALAC, and MP3 audio using software decoding.
To play multiple sounds with best performance, or to efficiently play sounds while the iPod is playing in the background, use linear PCM (uncompressed) or IMA4 (compressed) audio.
To learn how to check which hardware and software codecs are available on a device, read the discussion for the kAudioFormatProperty_HardwareCodecCapabilities constant in Audio Format Services Reference.
Both AAC and CAF formats work fine and offer decent file sizes. For certain background looping tracks I found MP3 files getting too big, but YMMV. Experimenting with a decent sound editing app is the only way to find the right balance between size and quality. I've had pretty good luck with Audacity and Amadeus Pro.
Suggest listening to the output with a pair of really good noise-isolating headphones on the device itself. Most people won't be listening to your stuff with these but as you decrease sound quality to shrink file sizes you'll start getting static and hum artifacts. It's just a matter of balancing size vs. quality and what you're willing to live with.
I use a combination of WAV files (for sound effects) and MP3 (for music), which seems to work fine. You can have trouble if you try to play multiple MP3 files at the same time - drop outs, or performance degradation, depending on your AudioSession settings.
If I had to compress my sound effects, I'm not sure which codec has the least decoding overhead. Something like Apple Lossless would likely work well, and would cut the size roughly in half.
I find mp3 fine, but keep in mind that decoding on the iPhone/Touch2G is only about 2.5x realtime speed.
I'm making an opengl game for iPhone. And I'm about to start adding sound effects to the app. I wonder what's the best framework for this purpose.
Is AV foundation my best option? Any others I'm missing, like Open AL perhaps?
General strength/weakness summary of iPhone sound APIs from a game perspective:
AVFoundation: plays long compressed files. No low-level access, high latency. Good for theme song or background music. Bad for short-lived effects.
System sounds: plays short (think 0-5 sec) sounds. Must be PCM or IMA4 in .aif, .wav, or .caf. Fire-and-forget (can't stop it once it starts). C-based API. Appropriate for short sound effects (taps, clicks, bangs, crashes)
OpenAL: 3D spatialized audio. API resembles OpenGL and is a natural accompaniment to it. Easy to mix multiple sources. Audio needs to be PCM (probably loaded by Core Audio's "Audio File Services"). Pretty significant low-level access. Potentially very low latency.
Audio Queue: stream playback from a source you provide (reading from file, from network, software synthesis, etc.). C-based. Can be fairly low-latency. Not really ideal for a lot of game tasks: background music is better suited to AVFoundation, shorter sounds to system sounds, and mixing to OpenAL or Audio Units. Can record from mic.
Audio Units: lowest public level of Core Audio. Extremely low latency (< 30 ms). C, and hard-core C at that. Everything must be PCM. Multi-channel mixer unit lets you mix sources. Can record.
Be sure you set up your audio session appropriately, meaning you declare a category that indicates how you interact with the rest of audio on the device (allow/disallow iPod playback in the background, honor/ignore ring/silent switch, etc.). AV Foundation has the Obj-C version of this, and Core Audio has somewhat more powerful equivalents.
Kowalski is another game oriented sound engine that runs on the iPhone/iPad (and OSX and Windows).
You might want to check out Finch, an OpenAL sound effect engine writter exactly with games in mind.