AHD Signal Transmission via Copper Cable
Updated: May 21, 2021
AHD | Full HD 1080p | 2k | 500 m | realtime HDTV video transmission
This article is written by Pipex Solutions - Johannes Dürrwaechter
Update August 5, 2020
The Principle of Transmission
AHD (Analog High Definition Video) is very suitable for switching from SD to HD technology. The signal is similar to an SD-CVBS signal, whereby the bandwidth of the signal is around 50MHz compared to 5.5MHz of the conventional SD signal. The relatively large bandwidth required, the effects of non-linear cable loss and other physical side effects represent the technical challenges for the development.
For the development we have been using the different, available camera modules from Skoopia. We have chosen symmetrical transmission via STP (shielded twisted pair) cables with a defined impedance, for reasons of better control of interference signals. The following different cable variants have been used during development and test:
Depending on the type and length of the transmission cable used, the camera signal is fed into the cable either directly (for lengths less than 30-50m) or via a line driver with high frequency pre-emphasis. With cable lengths in the range of 300-500m, the cable line driver is definitely necessary. At the end of the cable, the signal is then processed again so that a standard signal of 1 Vpp at 75 ohms is available.
The modules we have developed are a line driver high frequency pre-emphasis on the camera side and a module for reprocessing the camera signal at the end of the cable.
Line Driver for Camera Side
The line driver has the same dimensions as the Skoopia 21C14AT camera module.
Both modules (camera on the left, cable driver on the right) complement each other perfectly to form a functional unit. The ZIF cable of the camera can be plugged and fixed directly on the back of the line driver. The video signal and all other signals from the camera are available on a micro jack for transmission. The supply voltage for the camera module and amplifier is also connected via this socket.
Signal Conditioning at the End of the Cable
Since the assembly is not yet completely finished, we cannot show any pictures of it yet. However, the block diagram of the core module for signal processing is shown in the picture below.
The symmetrical video signal is fed to an adjustable attenuator, which protects the following stages against overload due to the high amplification required (up to 60dB). The signal is then converted into an unbalanced signal and fed to two adjustable gain amplifiers, one output of which goes directly to the adder and the other to the video equalizer. After adding the two signals, it goes to the clamp amplifier and then to the two outputs.
The entire circuit can be placed in a shielding cup with an edge length of 60 mm. The power supply for the amplifiers and the frequency setting of the equalizer is also required.
We initially intend to offer the module as a ready-to-use version with a power supply unit, so that the test can be carried out quickly and easily for manufacturers. The module can also be customized to fit it for installation, e.g. to optimize it for mounting inside a cable drum. The input range for the supply voltage is 9-36V.
All settings are made by computer with free software. This allows the complex settings to be saved to files and loaded again. They can also be saved in the module's EEPROM, with a total of 62 sets in memory available. Different cable lengths and types can be flexibly managed with these. The setting via computer also enables the programming of the parameters via USB / VCP or RS-485 interface if the circuit should be installed inaccessible.
The lab sample unit used to create the picture still had a 16-bit converter for the setting parameters, which explains the high numerical values. However, our tests have shown that a 12-bit resolution is sufficient for the series, which is also reflected in the price.
We have created all the parameter sets for our cable variants and they are freely available. As a service, we offer to support you in determining the cable parameters and parameterizing the signal processing.
Overall, it can be said that the extensive setting options cover the majority of all cases occurring in real live. Customer-specific additional setting options and / or extensions can of course also be implemented. The combination of the two modules allows reproducible, reliable video transmission of up to 500 m with high quality.
Read more details in the documents attached below.