PCM30

PCM30 referred to an application of the pulse code modulation (PCM) method described in which an analog signal is binary coded. For the application it is used for example when making calls.

The term is now often used as a synonym for the 30 * 64-kbit/s-Signal. This bit is also used in the first stage of the European PDH technique is therefore also referred to as E1 PCM30.

Originally it referred to a device in communications technology, which can convert 30 analog telephone signals into a digital bit stream of 2048 kbit / s.

The PCM30 system

The PCM30 basic system was / is used in digital switching equipment in Europe and served for the digital transmission of telephone traffic. It has 30 coder who can implement each one telephone channel into an 8 -bit digital word (byte ). The 30 bytes, together with a frame alignment word and an indicator channel, byte sent consecutively (multiplexer ). Thus, a 32-byte frame. In the receive direction, the 30 channels are distributed again by means of a demultiplexer 30 to the digital to analog converter; it arise analog signals again. The time slot 0 is used as a frame alignment word and than message word. These are alternately sent. Time slot 16 is used as an indicator channel for the voice channels. Each voice channel is assigned 4 bits which are transmitted sequentially in 16 frames, it results in a super-frame with a frequency of 500 Hz and a length of 16 ⋅ 32 ⋅ 8 = 4096 bits.

Structure of the PCM30 frame

Operation (short version)

The analog input signals are first limited by a low-pass. Thereafter, the signals are cyclically sampled by the multiplexer at a frequency of 8 kHz. The result is a PAM signal of all signals supplied. It is quantized by the A hörgerecht characteristics and binary-coded by an ' A / D- converter " in 8- bit word. Since 8000 every second samples are taken, each with 8 bits, it is produced in each case, a bit stream of 64 kbit / s Across all 32 channels (30 channels 2 administration telephone channels), the bit rate of 2048 kbit / s

For transmission, the binary signal is yet not suitable. Conducted transmission requires direct current freedom and constant synchronization information. The binary bit stream is re-encoded so via the intermediate stage of the AMI code in the HDB3 code. The entire bit stream of 32 telephone channels is HDB3 encoded at 2.048 Mbit / s transmitted to the remote peer. Distances greater than about 3.5 km can be bridged with regenerators, called the line equipment.

Since 2005, however no PCM30 links to more HDB3 base are provided in Germany. Instead, the 2B1Q coding is used, that the baud rate (Vs) is halved for the same transmission rate ( Vi ) and makes it possible to transfer the signal transmission in a lower frequency band. This in turn has a positive effect on the attenuation of the signal and its reach.

Data of the PCM30 system

• Number of channels ( time slots ): 32
• Number of voice channels: 30
• Frame duration: 125 microseconds
• Channel Time: 3.9 microseconds
• Kanalbitzahl: 8 bit
• Bit time: 0.488 microseconds
• Bitrate of 2048 kbit / s ( 2.048 Mbit / s)
• Bitrate per channel: 64 kbit / s
• Clock frequency: 8 kHz