Скачать или смотреть Understanding PCM Audio Frame Samples: What You Need to Know

A comprehensive guide to understanding PCM audio frame sample types, formats, and encoding, with emphasis on common configurations and characteristics.
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Exploring PCM Audio Frame Samples

When delving into audio programming, especially in the realm of Core Audio and Core Media, you may encounter challenges regarding the specifications of audio frames. One key concept that often needs clarification is the samples within Pulse Code Modulation (PCM) audio frames. In this guide, we will address a common question regarding the audio frame sample type in PCM: specifically, understanding the encoding of these samples based on provided audio parameters.

Understanding the Problem

In many audio applications, developers often work with CMSampleBufferRef buffers that contain audio data. This data is characterized by various parameters encapsulated in an AudioStreamBasicDescription, which specifies attributes such as sample rate, format flags, bytes, and channels. The confusion typically arises around the encoding details of the audio samples.

For example, consider an AudioStreamBasicDescription with the following properties:

[[See Video to Reveal this Text or Code Snippet]]

From this description, we need to decipher whether the audio sample size is a (short) integer, whether a frame consists of two shorts or one integer, and how to ascertain these details. Additionally, there is often speculation on whether the configuration implies a 32 bits per pixel (bpp) interleaved audio format.

Analyzing the Solution

To properly decode the above audio properties, it's crucial to examine the value of mFormatFlags. In our case, the mFormatFlags is noted as 0xe, which in binary is 0b1110. This binary representation allows us to determine several details about the audio stream's characteristics:

Breakdown of mFormatFlags

Big Endian (0b0010): This means the audio samples are stored in big-endian byte order.

Signed Integer (0b0100): The samples are represented as signed integers, which accommodates both positive and negative amplitudes in audio signals.

Packed (0b1000): The audio channels are interleaved, which means that audio samples from each channel are presented in alternate order.

This leads us to conclude that each frame consists of two interleaved samples of type int16_t (or 16-bit signed integers).

Sample Size and Structure

Given the parameters:

mBytesPerFrame: 4

mBytesPerPacket: 4

mChannelsPerFrame: 2

We can infer that:

Each channel takes up 2 bytes (since 4 bytes total divided by 2 channels results in 2 bytes per channel).

Therefore, each frame indeed contains 2 channels of audio samples, each represented as int16_t.

Conclusion on Encoding

From this analysis, we conclude that the audio frame consists of:

Two interleaved big-endian int16_t samples.

This configuration is common in stereo PCM audio and succinctly summarizes the encoding of the audio frame samples. Therefore, the assumption regarding the 32 bpp interleaved audio format is incorrect; instead, we are dealing with 16 bits per channel.

Final Thoughts

Understanding the characteristics and encoding of PCM audio frames is crucial for anyone working with audio data in programming. Armed with this knowledge, developers can handle audio buffers more confidently and streamline the audio processing tasks in their applications.

Feel free to dive deeper into the intricacies of audio processing, and don’t hesitate to ask questions related to your specific use cases!