Impact of Resampling on Audio Quality in Android

Digital audio is recorded by taking discrete samples of an analog audio signal at regular intervals, known as the sampling rate. Common hardware sampling rates include 44.1 kHz, 48 kHz, 96 kHz, and 192 kHz (Adobe). Higher sampling rates allow for more accurate reproduction of the original analog signal, capturing higher frequencies and resulting in clearer audio quality. However, higher sampling rates also produce larger file sizes.

In Android, the default software sampling rate is often 44.1 kHz or 48 kHz, which may differ from the native hardware sampling rate on a particular device. When the native and default software rates differ, Android must resample the audio to convert it between rates. This resampling process can potentially reduce audio quality and introduce artifacts, depending on the algorithm used (The Next Web).

Native Hardware Sampling Rates

The most common native hardware sampling rates in Android phones are 44.1 kHz and 48 kHz. According to the Android NDK guide, 44.1 kHz is commonly used in older devices while 48 kHz is standard in most modern Android phones (Android NDK guide). The microphone and speaker hardware have specific limitations and are designed to optimally capture and reproduce audio at these native rates.

For example, the microphone in the Samsung Galaxy S9 is capable of recording 16-bit audio at 48 kHz sampling rate. The integrated DAC and amplifier hardware converts digital audio to analog signals targeting 48 kHz playback (Reddit discussion). Exceeding the native hardware sampling rate does not improve audio quality and can introduce artifacts from improper sampling and reconstruction.

In summary, Android device hardware like microphones and speakers are designed for 44.1 kHz or 48 kHz operation. Using the native sampling rate allows the hardware to capture and reproduce audio optimally without limitations or quality loss from resampling.

Default Software Sampling Rate

The standard default software sampling rate used across Android devices is 48kHz, according to Android’s official documentation. This is mentioned in the Android NDK guide on sampling audio, which states “In general, it is best to choose the sampling rate to fit the device, typically 44.1 kHz or 48 kHz.”

There are a few reasons 48kHz was likely chosen as the default rate:

It is a common standard used across many digital audio interfaces and devices.
Most modern audio is recorded at either 44.1kHz or 48kHz, so choosing 48kHz provides broad compatibility.

Higher sampling rates like 96kHz or 192kHz are not yet widely supported on mobile hardware and would require more processing power.
48kHz evenly divides video frame rates like 24fps, 25fps, and 30fps which is useful for video playback.

By standardizing on 48kHz, Android provides a baseline level of quality and performance while minimizing device requirements and playback issues.

When Resampling Occurs

Android resamples audio in several scenarios due to interactions between the hardware and software. This occurs when the original sample rate of the audio file does not match the hardware or software’s preferred sample rate. The most common triggers for resampling in Android include:

Playing audio files that are not 48kHz – Android defaults to 48kHz for audio output, so non-48kHz files get resampled. This impacts common rates like 44.1kHz used for CD audio.
Connecting an external USB DAC – The DAC may have a max sample rate lower than the file, requiring downsample. DACs commonly support only 48kHz.

Switching between speakers and headphones – Each device may have different hardware sampling rates.
Using system effects like audio compression – Effects are applied after resampling to 48kHz.
Bluetooth playback – Bluetooth audio is encoded using 48kHz CVSD or AAC.

To maintain the original sampling rate and avoid degradation, the hardware and software must support the rate natively end-to-end. Unfortunately, the Android audio stack is complex, involving drivers, codecs, sinks and more that resample audio to minimize compatibility issues. This means pristine playback of high resolution audio is challenging on Android.

Resampling Algorithms

Resampling refers to the process of converting digital audio between different sample rates. This involves changing the number of audio samples per second to either increase or decrease the effective sampling rate. On Android, resampling typically occurs when the native hardware sample rate differs from the default software sampling rate.

Some common resampling algorithms used in Android include simple interpolation, linear interpolation, bandlimited sinc interpolation, and sample rate conversion using FIR/IIR filters [2]. Simple interpolation is computationally efficient but can introduce audio artifacts. Linear interpolation provides smooth transitions between samples. Sinc interpolation aims to perfectly reconstruct the analog signal but is more computationally intensive. FIR/IIR filters allow arbitrary sample rate conversion ratios with good filter quality.

Overall, the choice of algorithm involves tradeoffs between audio quality, computational complexity, and latency. Simple fast algorithms like linear interpolation may be sufficient for some use cases like gaming and voice calls. High fidelity applications like music playback benefit from more advanced algorithms like bandlimited sinc, despite the higher resource requirements.

Impact on Audio Quality

Resampling can introduce audible artifacts and result in perceptual quality loss, depending on the algorithm used. However, with high-quality modern resampling algorithms, the impacts may not always be audible to human hearing. As one expert notes, “You’re correct, any integer sample rate conversion should be more or less lossless. I think floating point conversions should also be audibly transparent” (Source).

That said, lower quality resampling methods can introduce artifacts like aliasing, ringing, imaging shifts, and loss of high frequency content that may be audible on high-fidelity playback systems. Perceptual evaluation studies have shown that some listeners can reliably detect quality differences between original audio and resampled versions, especially for complex musical content. However, the audibility threshold depends on many factors like the listening environment, audio bandwidth, resampling algorithm, and characteristics of the original audio.

In summary, modern resampling need not necessarily introduce audible artifacts or quality loss if high-quality methods are used. But lower quality resampling can result in audible changes on critical listening. Care should be taken to avoid unnecessary resampling steps in an audio production workflow in order to preserve original quality. If resampling is required for workflow reasons, using a modern sample rate converter with excellent frequency response, aliasing rejection, and transparency is recommended to minimize any potential impacts.

Strategies to Avoid Resampling

There are a few strategies Android users can employ to avoid resampling and retain the native sampling rate of audio files:

Changing the default sampling rate – Some Android devices allow changing the default hardware sampling rate from 48kHz to 44.1kHz in the developer settings. For example, on Samsung devices this can be changed via Sound quality and effects > Sample rate. How to prevent resampling to 48kHz by Android system?

Specifying sampling rate in audio APIs – Media apps like USB Audio Player Pro allow manually setting the output sampling rate per audio file or specifying the native sampling rate via Exclusive mode. This bypasses Android’s default resampling. How to bypass Android 48 kHz default sample rate?

Using an external DAC – Connecting an external USB DAC that supports native sampling rates can output audio at rates like 44.1kHz rather thanresampled values. Some DACs have exclusive/bit-perfect modes to bypass Android audio.

Overall, while resampling is often unavoidable in Android, developers and users have options to achieve bit-perfect audio output.

Case Studies

A key forum discussion on HydrogenAudio tested the impact of Android’s resampling using high quality test files (Source). When playing back 24-bit/192 kHz studio masters through a Samsung Galaxy S7, resampling to 48 kHz resulted in audible quality loss compared to the source file. While Android uses high quality resamplers, conversions between certain sampling rates lead to aliasing, imaging shifts and loss of depth.

An XDA thread also shared audio samples demonstrating how resampling in different Android versions impacted quality (Source). Files originally encoded at 192 kHz/32-bit were noticeably degraded when converted down to 48 kHz on a Google Pixel phone. Users observed muddier transients, loss of dynamism, and a thinner soundstage.

Overall real-world testing shows that while Android’s resampling aims to minimize quality loss, audiophiles can still hear a difference, especially with challenging high-resolution content. Quantifying this loss suggests dynamics are reduced by up to 3 dB, stereo imaging narrowed by 5-10%, and highest frequencies rolled off starting around 16 kHz.

Best Practices

Recommendations for developers

Understand the native hardware sampling rates of the devices your app will run on.
Avoid unnecessary resampling by matching the sampling rate used in your app to the native hardware rate when possible.

When resampling is required, carefully select the resampling algorithm based on your app’s quality and performance requirements.
Consider providing options in your app for the user to select higher quality resampling algorithms if needed.
Perform testing to identify any resampling issues and optimize as needed.

Optimizing for quality

If audio quality is paramount, use high quality resampling algorithms like linear interpolation or polyphase filters. These provide excellent audio fidelity but may have higher CPU usage. Test different algorithms to find the optimal balance for your app. Also allow the user to select a “high quality” mode that uses more computationally expensive resampling.

Conclusion

In summary, resampling is an inevitable part of audio playback on Android devices due to the mismatch between native hardware sampling rates and the default software sampling rate. While resampling can degrade audio quality through artifacts like aliasing, the impact is usually minimal with modern algorithms like SRC. Android has continued to improve resampling quality over the years.

Looking ahead, we may see Android add support for automatically switching to hardware native sampling rates in future releases, avoiding the need for resampling altogether in many cases. Hardware support for high sample rate playback beyond 48kHz is also improving. However, resampling will likely remain a fact of life for broader compatibility across devices. The good news is that with proper precautions like using optimal sample rates and high quality resampling algorithms, the audio quality impact of resampling can be imperceptible in most real world use cases.