Android system has risen rapidly, surpassing iOS and Symbian to become the largest intelligent device operating system. Its market share is rapidly expanding. There will be a large number of multimedia devices adopting this system, so Android is suitable for use as an operating system for audio and video equipment. What? Let's take a look at the audio architecture of Android today.

Google Android

Android based on Linux, let us first understand the characteristics of Linux. Linux uses ALSA as its audio architecture, the full name of the Advanced Linux Sound Architecture, which means the advanced Linux sound architecture. After the 2.6 core, ALSA became the default audio sub-architecture for Linux systems. Replaced the previous OSS Open Sound System, an open sound system.

Linux ALSA audio architecture

ALSA is not very easy to understand, it is first a driver library, contains a large number of open source drivers for sound card devices, and provides a core layer API to communicate with the ALSA library, while the ALSA library is the middle layer for applications to access and manipulate audio hardware. This middle layer has a standard interface, developers can develop without regard to hardware differences, it is very helpful to improve development efficiency. ALSA can be backward compatible with OSS because OSS has been eliminated and its compatible working mode is not discussed.

ALSA library files under Android

This system has been inherited into Android. Before Android2.2 [including 2, 2], a file of LibAudioALSA.so can be found in the system folder. This is the ALSA library file, which is called by other applications to communicate with the sound card device. Android audio architecture is not fundamentally different from Linux.

In the desktop version of Linux, in order to be compatible with all kinds of sound cards, Linux also sets a SRC [Sample Rate Converter] link, which forces the SRC to 48 kHz output when the current sampling rate is lower than 48 kHz. This SRC link is located in the mixer section of the ALSA plug-in module. Android has improved on this.

Android has added an AudioFinger, which can be easily understood as a standardized plug-in module for Android's ALSA audio subsystem. It includes sub-modules such as AudioMixer, AudioResampler, and AudioResampler, which we understand. Android has changed to a new name. For SRC, Android has been improved, but the improvement is not to remove the SRC, but to modify the default output frequency, Android SRC target sampling rate is 44.1kHz, the sampling rate of this value will be processed by SRC. For example, playing a signal with a sampling rate of 48 kHz, the final output is 44.1 kHz, which will have a negative impact on the sound quality. This can be proved by testing.

Meizu Meizu M9 smartphone - frequency scanning, recording terminal gain 20dB

Meizu Meizu M9 smartphone - 48kHz frequency scanning, recording terminal gain 20dB

Comparing this set of results, we can see that the SRC is destructive to sound quality. This problem is not just the existence of Meizu M9. It exists in almost all Android devices.

Huawei Huawei U8800 Smartphone - 48kHz Frequency Scan

Lenovo Lenovo LePad Tablet - 48kHz Frequency Scan

Malata Malata Zpad T8 Tablet - 48kHz Frequency Scan

ALSA is an audio architecture designed for the Linux desktop version. It is actually not suitable for smart terminal devices. At least a lot of open source driver code can be removed. For Android, these are all waste code. Starting with Android 2.3, a new audio architecture has been enabled. It gave up the ALSA architecture that has been used, so there is no longer the LibAudioALSA.so file in the system folder.

There is no ALSA library file under the system folder of Android2.3.

The audio architecture after Android 2.3

Since Android 2.3, the architecture has been modified, optimized for internal code, removing redundant code, theoretically making the system more efficient, and can understand the new architecture as a streamlined or customized for smart devices. ALSA architecture. Unfortunately, it also has serious problems with SRC degradation, which can be proved by testing.

HTC HD2 @Android 2.3 Smartphone - 48kHz frequency scan

Tests can be found that the new architecture of Android 2.3 does not have a positive effect on sound quality.

ASUS ASUS Eee Pad Transformer TF101 Tablet - 48kHz Frequency Scan

Android 3.0 is designed specifically for tablets, and the audio and video experience has become more important. Is the new system going to have new progress in sound quality? The test results are still disappointing.

The Android system will have the same sampling rate of 44.1 kHz output, which causes many restrictions. It will not be able to achieve good playback of 96 kHz, 192 kHz high-definition audio programs. A large number of video programs are derived from DVD or Blu-ray discs. The adoption rate is 48 kHz, Android devices. When playing back these video programs, the sound quality will be greatly reduced.

In theory, the software SRC can improve the sound quality by replacing the algorithm, but it is not realistic. The CPU used by the intelligent terminal is mostly ARM. The floating point computing power of the ARM chip is limited, and the SRC requires a large amount of floating-point computing resources, even if With the high-quality SRC algorithm, its operation is also at the expense of device performance and power consumption, and the utility is poor.

From the audio architecture and process analysis of Android, it can be considered that when playing a music program with a sampling rate of 44.1 kHz, SRC will not be triggered, and the sound quality can be guaranteed, which is theoretically true. But it also has problems. Whether it is the previous ALSA architecture or the improved architecture after Android2.3, its driver library is located at the core layer, which means that audio equipment manufacturers and users cannot install drivers to improve the sound quality like the PC platform. The actual test also shows that the sound quality of Android devices is generally biased, and Soomal has a lot of tests to prove.

We turn our attention to iOS, iOS is very closed, we can't even know the specific structure of its architecture, but iOS devices do not have the problem of hardware device diversity, so it is easier to achieve better sound quality. iOS can achieve targeted development and improvement to achieve better sound quality. The actual situation is also the case. So far, there is no sound quality of an Android device comparable to any iOS device. This gap, we think is not from the hardware, but the operating system.

The limitations of Android audio architecture also make it difficult to become a high-quality audio and video platform. If you want to design a high-definition video player based on Android, the first thing you need to do is not to design the hardware, but to modify the existing architecture, or Simply design a dedicated architecture to replace the common architecture of Android. From source code analysis, Android and native Linux underlying can support various sampling rates, and open source also makes it have a transformation basis. Therefore, in the hands of companies with strong technical strength, Android can also become a phoenix.

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