What is Android audio format?

Android provides a rich audio API and framework for apps to work with audio. Audio is an essential component in many apps, from music players to video streaming to games. Android supports standard audio file formats and codecs and provides APIs for audio playback, recording, and management.

The Android audio architecture consists of several layers including applications, framework APIs, native codec libraries, and the audio hardware. Apps can leverage high-level Java APIs or low-level C/C++ audio APIs in the Android NDK.

Key audio capabilities supported in Android include playback and recording in common formats like MP3, AAC, OGG, MIDI, Wave, and more. Android manages audio focus and routing between multiple apps. Effects, tuning, and audio-driven haptic feedback are also available through Android audio APIs.

Audio is integral to delivering immersive user experiences in mobile apps. The Android audio framework enables developers to build robust audio features across a range of app categories and devices.

Common Audio Codecs

Here are some of the most common audio codecs supported by Android:

MP3 – MPEG-1/2 Audio Layer 3 is a lossy compressed audio format. It is one of the most popular codecs for music playback and supports bitrates ranging from 8-320 kbps. MP3 files use the .mp3 extension. [1]

AAC – Advanced Audio Coding is a lossy compressed format that was designed to be the successor to MP3. AAC generally achieves better sound quality than MP3 at the same bitrate. It supports up to 48kHz sample rates. AAC files use the .m4a extension on Android. [2]

FLAC – Free Lossless Audio Codec is an open source lossless audio format. FLAC files are typically 50-60% the size of uncompressed audio and supports up to 24-bit/192 kHz. The .flac extension is used.

OGG – Ogg Vorbis is an open source audio format alternative to MP3 and AAC. It is also a lossy compressed format but is free of licensing fees. Ogg files use the .ogg extension.

MIDI – Musical Instrument Digital Interface is a technical standard for representing sheet music. MIDI files contain musical notes, timing, and volume information but do not contain the actual audio waveform data. MIDI uses the .mid extension.

[1] https://developer.android.com/media/formats/audio-codecs
[2] https://www.androidauthority.com/bluetooth-audio-codecs-997074/

Android Audio API

The Android Audio API provides the fundamental building blocks for working with audio in Android apps. The key audio classes include AudioTrack, AudioRecord, and AudioManager.

AudioTrack is used for playing audio. It allows you to write audio data to a buffer which is then output as an audio signal. You can specify attributes like sample rate, channel configuration, encoding, etc. AudioTrack enables low latency audio playback.

AudioRecord is the counterpart to AudioTrack used for recording audio from the microphone or other input source. You configure the recording properties like sample rate and call read() to capture audio data into a buffer. AudioRecord provides low latency audio capture.

AudioManager provides access to volume and audio routing control. You can use it to adjust the volume, set the speakerphone mode, handle audio focus, and more. AudioManager allows you to integrate with overall audio policies.

Together these core classes allow fine-grained control over audio input, output and configuration in Android apps. They form the basis for implementing high-quality audio capabilities.

Audio Encoding

Android supports encoding audio into various compressed and uncompressed formats using the MediaCodec API. The most common audio codecs supported include:

• MP3 – A very common lossy compressed format.
• AAC – Another popular lossy compressed format, especially for streaming.
• FLAC – A lossless compressed format that preserves audio quality.
• PCM – The standard uncompressed format used for raw audio data.

To encode audio using MediaCodec, you need to follow these basic steps:

1. Create a MediaFormat object specifying the desired codec, sample rate, channel count etc.
2. Configure the MediaCodec encoder using createEncoderByType()
3. Feed raw PCM audio data into the encoder using the queueInputBuffer() method.
4. Dequeue encoded buffers using dequeueOutputBuffer().
5. Write the encoded data to file or send over the network.

Here is some sample code demonstrating encoding raw PCM to AAC:

https://developer.android.com/reference/android/media/MediaCodec#encodeaudiotoaac

By following this overall process, you can efficiently encode audio into compressed formats optimized for storage and transmission. Make sure to check the official Android developer guides for more details and codec-specific instructions.

Audio Decoding

Android provides several ways to decode audio files and play the decoded audio data. The main classes used for audio decoding are MediaPlayer, SoundPool, and MediaCodec.

MediaPlayer is used to play audio files directly, handling both decoding and playback internally. It supports common formats like MP3, AAC, WAV, FLAC, etc. To use MediaPlayer, you create an instance, set a DataSource pointing to the audio file, prepare() the player, and call start() to begin playback. MediaPlayer is easy to use but less flexible than the lower-level options.

SoundPool is used for short audio clips that need to be played simultaneously, like sound effects in a game. You load each audio clip into the SoundPool using load(), then call play() as needed, passing the sound ID. SoundPool handles concurrent decoding and mixing of sounds efficiently.

For more advanced use cases, MediaCodec can be used to perform audio decoding separately from playback. You configure a decoder for the specific audio format, then feed it encoded audio data using queueInputBuffer(). The decoder outputs raw PCM audio frames that can then be played back or further processed. Using MediaCodec directly requires more work but provides finer control over decoding.

In summary, Android provides multiple audio decoding options depending on your needs, from simple playback using MediaPlayer to advanced low-level control with MediaCodec.

Audio Playback

Efficient audio playback is critical for delivering a good user experience in Android apps. Here are some best practices:

Use streaming with buffering to playback long audio files. Download an initial chunk, begin playback, and stream the rest of the file¹. Monitor the buffer to avoid underruns.

Use the AudioTrack API for direct and efficient audio playback. Initialize it with appropriate attributes like sample rate, channel config, audio encoding².

Minimize playback disruptions by handling audio focus changes, ducking, and pausing/resuming correctly when needed³.

Adjust buffer size, use setVolume, or process audio before playback to optimize quality.

Test playback on various devices to ensure consistent behavior.

Audio Recording

Android devices allow capturing audio and recording it as digital files for later playback or sharing. The primary methods for recording audio on Android involve using the built-in microphone to capture audio and save it in a supported format.

The simplest way to record audio is through the Recorder app included on many Android devices. The Recorder app provides basic functionality to start, stop, and save recordings using the device’s microphone. Files are typically saved in AMR, AAC, or MP3 format depending on the device and version of Android.

For more advanced audio recording, Android provides the AudioRecord class as part of the Android audio API. This allows recording raw PCM audio data from the microphone and then encoding it into a compressed format like MP3 in the app. The AudioRecord class provides low-latency access to the microphone for capturing audio in real-time. Apps can customize parameters like sample rate, channel count, and encoding for their audio recording needs.

Some third-party apps provide additional features like trimming recordings, inserting bookmarks, transcription, and more advanced controls. But the built-in Recorder app or AudioRecord class offer simple, native options for capturing audio through the microphone on most Android devices.

Audio Focus

Audio focus in Android refers to how the audio system handles playback from multiple apps that want to output sound at the same time. The core goal is to ensure a good user experience by preventing audio from different apps from playing over each other in a confusing or unpleasant way.

The AudioManager class provides interfaces for requesting and handling audio focus. When an app requests audio focus, it essentially asks the system for exclusive access to play audio. If another app already has audio focus, the new requester will be denied. This allows smooth transitions between audio sources and prevents audio clashes.

Apps can request different audio focus behaviors like gaining focus temporarily or indefinitely. The system manages transfer of focus between apps based on priority and duration of requests. Apps should always check if a focus request succeeds before playing audio. A focus gain request could be denied if another higher priority app holds focus.

When an app loses audio focus, the system gracefully fades out its audio before stopping playback. The app is notified so it can pause audio until focus returns. This results in a seamless hand-off of audio output between apps.

Proper use of audio focus is important for delivering good user experiences. As the Android media documentation states: “Misuse of audio focus can lead to conflicts between audio sources and confusing user experiences.” By thoughtfully managing shared audio resources, multiple apps can play nicely together.

Audio Effects

Android provides a framework for applying audio effects to audio streams via the AudioEffect API. Developers can use built-in audio effects like bass boost, virtualizer, equalizer, reverb, and presets to enhance audio playback quality.

Some common audio effects applied in Android apps include:

• Reverb – creates a spacious auditory perception, simulating a large room
• Equalizer – boosts or attenuates different frequency bands
• Bass boost – amplifies low frequencies for a richer sound
• Virtualizer – widens the stereo image for headphone listening

Developers create an AudioEffect instance, specify the audio session ID to attach it to, and call the effect-specific methods to activate the effect. Multiple effects can be chained together on the same audio session. Some parameters can also be adjusted in real-time.

There are also many third party Android apps like Smart Audio Editor that provide GUIs for applying customizable audio effects to recordings and music playback.

Conclusion

In summary, Android offers robust support for working with audio through its Audio API and common audio codecs like MP3, AAC, FLAC, and more. Key capabilities include audio playback, recording, focus, and effects. Android also enables high-quality lossless audio output over USB-C on supported devices. This unlocks studio-quality listening experiences on mobile.

Looking ahead, the future of Android audio is bright. We can expect continued enhancements to audio quality, personalization through AI, and immersive experiences with haptic feedback. Lossless Bluetooth audio and more advanced audio effects are also on the horizon. The Android platform will remain at the forefront of mobile audio innovation and empower developers to build the next generation of audio experiences.