PCM AUDIO,PCM Format Audio file detailed

PCM file: Analog audio signal is directly formed by the analog-to-digital conversion (A/D transform) binary sequence, the file has no additional file header and file end flag. The Convert tool in Windows can convert PCM audio format files into Microsoft WAV format files.

Digitizing the audio is actually about digitizing the sound. The most common way is to modulate the PCM (pulse code modulation) with a pulsed code. Operating principle is as follows. First we consider the sound passing through the microphone and converting it into a series of signals of voltage change, as shown in Figure A. The horizontal coordinates of this map are second and the longitudinal coordinates are voltage-sized. The way to convert such signals into PCM format is to use three parameters to represent the sound, which are: number of channels, number of sample bits, and sampling frequency.

Sampling frequency: The frequency of sampling, which is the number of times a sound sample is obtained per second. The higher the sampling frequency, the better the sound quality, and the more realistic the restoration of sound, but at the same time it accounts for more resources. Because of the limited resolution of the human ear, too high a frequency can not be distinguished. In 16-bit sound card has 22KHz, 44KHz and other levels, of which, 22KHz equivalent to the ordinary FM broadcast sound quality, 44KHZ is equivalent to CD quality, the current sampling frequency is not more than 48KHz.

Number of sample bits: That is, the sampled or sampled values (that is, quantifying the sampled sample amplitude). It is a parameter used to measure the change in sound fluctuation, and it can be said to be the resolution of the sound card. The larger the value, the higher the resolution, and the more powerful the sound.

Number of channels: very good understanding, there are mono and stereo points, mono sound can only use a speaker voice (some also processed into two speakers output the same channel sound), stereo PCM can make two speakers are audible (generally left and right channels have division of labor), more can feel the space effect.

The following illustration is used to see the concept of sample bits and sampling frequency. Let's take a look at these pictures. The black curve in the figure represents the natural sound waves recorded by the PCM file, the red curve represents the sound waves from the PCM file output, the horizontal axis is the sampling frequency, and the ordinate is the number of sample bits. The grids in these pictures are gradually encrypted from left to right, first increasing the density of the horizontal axis, and then increasing the density of the ordinate. Obviously, the smaller the horizontal axis, the smaller the interval of two sampling time, the more favorable the real situation of the original sound, in other words, the greater the frequency of sampling, the better the quality of the sound; Similarly, the smaller the ordinate units, the better the sound quality, i.e. the larger the number of samples is better.

The number of samples in the computer generally have 8 and 16 points, but one point please note that 8 is not to say that the ordinate is divided into 8 parts, but divided into 2 8 or 256 parts; the same 16 bits are 2 copies of the ordinate into 16, and the sampling frequency is usually 11025HZ (11KHz), 22050HZ (22KHz), 44100Hz (44KHz) three kinds.

Sample point T1 T2 T3 T4 T5 T6 T7 ... t16 t17 t18 t19 T20

Amplitude 0011 01010111 1001 1011 1101 1110 ... 0110 0110 0101) 0011 0000

So now we can get the formula for the capacity of the PCM file:

Storage = (Sample frequency * Sample bit * Channel) * Time/8 (in bytes)

Bit rate of waveform sound = sample Frequency * Quantization number of bits * channel number

For example, the standard sampling frequency of the digital CD (CD-DA, Red Book standard) is 44.lkHz, the sampled digits are 16-bit, the stereo (2-channel), can be broadcast almost undistorted frequency up to 22kHz of sound, which is the highest frequency sound that humans can hear. Laser turntable a minute of the amount of music you need to store:

(44.1*1000*l6*2) *60/8=10,584,000 (bytes) =10.584mbytes

This value is the amount of disk space that the PCM sound file occupies on the hard disk.

The format of the computer audio file determines the quality of its sound, in daily life, telephones, radios, etc. are analog audio signals, that is, there is no concept of sampling frequency and number of sampling bits, we can compare this:

44khz,16bit's voice is called: CD quality;

22KHz, 16Bit sound effect is similar to the stereo (FM Stereo) broadcast, called: Broadcast sound quality;

11kHz, 8Bit of Sound, called: telephone quality.

The WAV file of Microsoft is a kind of PCM code

"Original link"

[1] shuyong1999 's column, "Audio PCM"