How Do Broadcasters Convert Analog to Digital Signal?

In a multimedia system, when the signals at the input and output terminals are analog signals, since the system transmits and stores digital signals, the analog signals need to be digitized.

In a multimedia system, when the signals at the input and output terminals are analog signals, since the system transmits and stores digital signals, the analog signals need to be digitized.

Digital concept of analog signal

Analog signals refer to information expressed by continuously changing physical quantities (time, amplitude, frequency, phase, etc.). Analog signals are widely distributed in every corner of nature, such as the daily temperature, the speed of the car during driving, and the voltage amplitude of a node in the circuit.

Digital signals are artificially abstracted discontinuous signals, which can usually be obtained from analog signals. The value of the digital signal is discontinuous, and the number of values is limited.

Analog signal

Digital signal

Digitizing an analog signal is to convert the analog signal into a discrete sequence that can be represented by a limited number of values. But what exactly is this process? The figure below gives us the answer.

Digitization of analog signals

Digitization of analog signals

Analog audio signal is converted into digital audio signal: Analog audio signal is a signal that is continuous in time and amplitude. Its digitization process is as follows.

1. Sampling: Digitize the signal on the time axis. That is, the value of the analog signal is extracted at a fixed time interval. In this way, a time-continuous information wave can be converted into a discrete signal with a limited number of values in time after sampling.

2. Quantization: Digitize the signal on the amplitude axis. That is to say, a finite number of amplitude values are used to approximately restore the original continuously changing amplitude value, and the continuous amplitude of the analog signal is changed to a finite number of discrete values with a certain interval.

3. Encoding: Represent the quantized value (decimal number) of each sample with a binary number.

Digitization of analog signals

Analog video signal is converted into digital video signal: Analog video signal is a signal that is continuous in space and grayscale. Its digitization process is different from audio signals.

1. Sampling: Digitize the signal in space. That is, the gray value of some points in space is used to represent the image. These selected points are called samples (or pixels). When sampling, the total number of pixels in the portrait and landscape orientation of the image will determine the quality of the digital image. Unlike audio signals that sample amplitude values in the time domain, video signals sample functions that represent images. The sampled image is segmented into spatially discrete pixels, but its gray levels are still continuous.

1. Quantization: digitize the signal in grayscale, and convert the pixel grayscale into discrete integer values.

The number of different gray values in a digital image is called gray level. For example, in the RGB888 (R: red, G: green, B: blue) color mode, the number of each color is 256, so the gray level is 256. 256 = 2, so each color can be represented by an 8-bit binary number, which quantizes the image information into a binary number. (From the perspective of visual effects, a grayscale image with a quantization of 6 bits or more is satisfactory visually, because the human eye can generally distinguish up to 100 grayscale levels.)