I'm wondering whether it's possible to slow down a sound in xcode. I mean I'll add some .mp3 file in my supporting files in xcode and I'll create app which will be able to speed it up or slow down. For example with slider. Is it even possible? If yes, could anyone help me with some idea? Thanks
AVAudioPlayer has a rate property which should be able to help you accomplish your goal.
http://developer.apple.com/library/IOS/#documentation/AVFoundation/Reference/AVAudioPlayerClassReference/Reference/Reference.html
The audio player’s playback rate. #property float rate
Discussion
This property’s default value of 1.0 provides normal playback rate.
The available range is from 0.5 for half-speed playback through 2.0
for double-speed playback.
To set an audio player’s playback rate, you must first enable rate
adjustment as described in the enableRate property description.
I also found a good SO post on the AVAudioPlayer's rate:
AVAudioPlayer rate
Seems like as you'd mentioned you could set a slider with values from 0.5 to 2.0 and on valuechanged modify the audio players rate by using
- (IBAction)changeValue:(UISlider *)sender
{
//made up assumed ivar names
if ([_audioPlayer respondsToSelector:#selector(setEnableRate:)])
_audioPlayer.enableRate = YES;
if ([_audioPlayer respondsToSelector:#selector(setRate:)])
_audioPlayer.rate = [NSNumber numberWithFloat:slideValue];
}
Playing PCM audio at a faster or slower rate than its sample rate changes its pitch and also introduces considerable artefacts. If you're OK with this, the approach you would use is to decode the MP3 into PCM audio and then use Direct Digital Synthesis Oscillator to control the playback rate.
If you want to maintain pitch but change speed, you need a audio time-stretching algorithm.
Dirac3 from DSP Dimension is a commercial product that can do it, and which is available for licensing for use in iOS application. Other commercial solutions exist.
DSP Dimension's blog provides a helpful tutorial on the basics of how to implement pitch-shifting
using a FFT. Time stretching is essentially the same process. However, there's a fair bit of secret sauce in the DIRAC plug-in they don't tell you about.
Be warned that unless you're a Electronics engineering, physics or maths graduate, you'll probably it tough going to fill in the blanks.
Related
Decreasing the playback speed of AudioPlayer severely decreases the quality of the audio being played; the audio becomes very "noisy".
Is there any way to fix this or is it an issue with the just_audio implementation?
Reproduce:
final AudioPlayer player = AudioPlayer(); // Create audio player
player.setAsset("..."); // Load audio file
player.setSpeed(0.5); // Halve speed
player.play(); // Start playing
Just to preface this answer, time stretching is a difficult thing to do in real-time because it has to stretch time without stretching the sound waves (stretching the sound waves would lower the frequency and hence the pitch, so it has to stretch time while filling the gaps with fabricated extensions to the existing sound waves). As a result, the very best real time algorithm will still introduce artifacts and distortions.
Now to answer your question, just_audio doesn't provide any options to change the time stretching algorithm, but it does use the best available algorithms for each platform, for general purpose usage. The Android implementation uses Sonic which is better quality than Android's own built-in algorithm. On iOS/macOS, AVAudioTimePitchAlgorithmTimeDomain is used which seems to produce the least distortion at speeds below 1.0 out of the different algorithms Apple provides, although newer iPhones/iOS versions may produce higher quality output. On web browsers, it uses whatever algorithm that web browser provides.
If you need to try out alternatives, you would need to make a copy of just_audio and edit the code that selects the algorithm. You are unlikely to find better options for Android and web, but you might like to experiment with the different iOS/macOS algorithms by searching for AVAudioTimePitchAlgorithmTimeDomain in the code and changing it to one of the other options listed in Apple's documentation. You may find one of the other algorithms works better if you have a specialised use case.
I'm not sure if it's allowed ask these questions here, but looks important for us webdevelopers (even bad dev like me :p ).
The question is about export setting videons on Premiere. I'm looking for a background video 30s like airbnb or paypal. Yesterday, I check paypal size and it's only 10/15 Mb for more than a minute. How did they do?
Obviously you want a low average bit rate. Things that can help with that are: keep the resolution low (you can scale it up a bit on the client); use H.264 High Profile (for the H.264 version); use 2-pass encoding; use variable bit rate. You can try increasing the GOP length too.
I assume there's no audio, so that shouldn't be an issue. (Can't remember if Adobe has an option for no sound track, but you can set the audio to a very low bit rate, or post-process it with ffmpeg or something to remove the audio track.)
If you have any control over the video content, you can try to keep it compressible. For example, avoid video with lots of detail or rapid motion. You might be able to selectively blur parts in a way that doesn't look bad. If it doesn't move too fast, you might be able to decrease the frame rate.
If you really want to optimize, you'll probably need to experiment a lot.
I am right now working on one application where I need to find out user's heartbeat rate. I found plenty of applications working on the same. But not able to find a single private or public API supporting the same.
Is there any framework available, that can be helpful for the same? Also I was wondering whether UIAccelerometer class can be helpful for the same and what can be the level of accuracy with the same?
How to implement the same feature using : putting the finger on iPhone camera or by putting the microphones on jaw or wrist or some other way?
Is there any way to check the blood circulation changes ad find the heart beat using the same or UIAccelerometer? Any API or some code?? Thank you.
There is no API used to detect heart rates, these apps do so in a variety of ways.
Some will use the accelerometer to measure when the device shakes with each pulse. Other use the camera lens, with the flash on, then detect when blood moves through the finger by detecting the light levels that can be seen.
Various DSP signal processing techniques can be used to possibly discern very low level periodic signals out of a long enough set of samples taken at an appropriate sample rate (accelerometer or reflected light color).
Some of the advanced math functions in the Accelerate framework API can be used as building blocks for these various DSP techniques. An explanation would require several chapters of a Digital Signal Processing textbook, so that might be a good place to start.
I know how to play mp3 files and whatnot in Xcode iOS. But how do I play a certain frequency, like if I just wanted to emit a C# note for 25 seconds; how might I do that? (The synth isn't as important to me as just the pitch of the note.)
You need to generate the PCM audio waveform that corresponds to the note you want to play and store that into a sample buffer in memory. Then you send that buffer to the audio hardware.
Here is a tutorial on generating waveforms of several types. The article goes into some details on the many aspects to a note you need to consider, including the frequency, volume, waveform shape, sampling rate, etc. The article comes with Flash source code, I think you should have no problem taking the concepts and adapting them to iOS.
If you also need a library that you can use to play the generated buffers on iOS, then I recommend the open source Finch.
I hope this helps!
You can synthesize waveforms of your desired frequency and feed them to the callbacks of either the Audio Queue or the RemoteIO Audio Unit API.
Here is a short tutorial on some of the code needed to create sine wave tones for iOS in C.
i want to detect heart rate using iphone sdk does someone knows any method for calculating heartbeat rate?
Fast Fourier Transform is a class of algorithms that can quickly turn samples into an analysis that tells you how prominently ceratin frequencies occur in that sample. For more check out:
Wikipedia: FFT
Literate program example: Cooley-Tukey FFT
This is relevant to your problem because: (1) heart rate is itself a frequency, and (2) most of the sound that comes through the body that you can measure will be within a certain frequency range. Dropping frequencies outside this range means dropping all or mostly noise.
Good luck!
Well I've seen various implementations. Some of them use the accelerometer to detect minute movements in your arm/hand when you hold the phone, some of them can use the microphone, you could also do a manual 'tap' interface where you tap the screen while checking your own pulse.