Cable TV analog signal
Cable TV analog signals (the traditional 'antenna' signals) were the mainstream signal form of early cable TV. Essentially, it directly 'carries' TV programs (images and sounds) on continuously changing electrical signals and transmits them to users' homes through coaxial cables.
Key features:
1. No set-top box is required; old CRT TVs can watch by directly plugging in the coaxial cable, with 'plug and play' signals;
2. The signals are continuous, and the changes in images and sounds are 'smooth'; for example, transitions in actions and sound levels have no 'jumpy feeling';
3.There are only few channels, usually dozens, the picture quality is easily affected such as 'snow' during rainy days or when the cable is aging, and there is no interactive function, you can only passively watch live broadcasts and cannot request on-demand or rewatch.
Cable TV digital signal
Cable TV digital signal (currently mainstream 'set-top box' signal) is the standard signal format for cable TV today. Essentially, it converts television programs (images, sound) into a '0/1 binary sequence', compresses it, and transmits it through cables. Users need to use a 'set-top box' to decode it in order to watch.
Key features:
1. A set-top box is required; the signal is 'encrypted discrete data' that must be decrypted and decoded by the set-top box to restore it to a displayable image/sound on television;
2. The signal is discrete, with images and sounds broken down into 'data blocks' for transmission, for example, 1 second of video is divided into 25 frames (each frame is a group of binary data);
3. Enhanced experience, a large number of channels (usually several hundred, including HD channels), clear picture quality (no snowflakes, unless signal interruption leads to 'buffering/black screen'), and supports interactive features (on-demand movies, replay missed programs, pause live broadcasts).
| CATV analog signal | CATV digital signal | |
|---|---|---|
| Receiving device | No set-top box required, directly connect to the TV (old TVs come with analog signal ports). | Must have a set-top box (the set-top box is responsible for decryption and decoding), new smart TVs can have a built-in digital decoding module. |
| Number of channels |
30-50 channels, only basic ones |
100-500 sets, including HD and professional channels like sports and film channels |
| Picture quality / Sound quality | Prone to interference (snow, ghosting, noise), low resolution (like 480P). | No interference (clear and stable), supports HD (1080P), 4K, and stereo sound quality. |
| Interactive features |
None (only passive viewing of live broadcasts, no fast forward or rewind). |
playback, pause, program scheduling, even gaming/shopping |
| Signal transmission efficiency |
Low (1 cable can only transmit 1 set of analog channels, resource waste). |
High (through 'signal compression', one cable can transmit dozens of digital channels). |
| Anti-interference ability |
Weak (cable aging and external electromagnetic interference directly affect the image). |
Strong (interference does not affect the judgment of 0/1, only severely will it stutter/blackout). |
Signal Conversion in Cable Television
The signal conversion in CATV is not a "single step," but rather a complete process that runs through "program production → transmission → user reception," with the core being the conversion from "analog to digital" at the broadcasting end and from "digital to analog" at the user end.
Step 1:
At the Television Station (analog signal → digital signal) - Signal "packaging and compression"Programs produced by television stations (such as news captured by cameras and live broadcasts from studios) are initially in analog signals (continuous images/sounds) and must be converted into digital signals for efficient transmission.
1. A/D Conversion: Using professional equipment (such as video encoders) to "sample, quantize, and encode" the analog images/sounds (consistent with the general A/D principles discussed earlier), for example, converting an analog image of 25 frames per second, with each frame sampled into 1920×1080 pixels, and then quantized into binary data;
2. Compression Encoding: The digital signal is large in size, requiring compression using standard formats (like MPEG-2, H.264) (for example, by removing "duplicate pixels" in images, such as static backgrounds) to reduce transmission bandwidth;
3. Encryption and Multiplexing: The multiple sets of compressed digital channels are "packed" into one transmission link, while also being encrypted (to prevent reception by unpaid users), and then transmitted via coaxial cables or optical fibers to the user's neighborhood.
Step 2:
User Side (Digital Signal → Analog / Digital Signal) - Signal "Decryption and Restoration" The user receives an "encrypted digital signal" which needs to be converted by the set-top box to adapt to different televisions.
1. Decryption and Demultiplexing: The set-top box inserts a "viewing card" to decrypt the signal, then selects the desired channel from the "packaged multiple channels" (demultiplexing);
2. Digital-to-Analog Conversion (D/A): If the user has an old analog TV (without digital decoding capability), the set-top box will convert the decrypted digital signal into an analog signal (a continuous electric signal), which is transmitted to the TV through "AV cables" (red, yellow, and white cables) for direct viewing;
3. Direct Output of Digital Signal: If the user has a smart TV (with built-in digital decoding module), the set-top box can directly output the digital signal (via HDMI cable), allowing the TV to display high-definition images directly (skipping the D/A step).
