Out in Front: November 23, 1995
Broadcom recently partnered with Hewlett-Packard Co to produce Broadcom's BCM3115 QAMLink digital-transmission chip for digital-TV set-top boxes. The dual-channel receiver offers forward error correction (FEC) and integrates 64- or 256-quadrature-AM (QAM) demodulation, concatenated Viterbi/Reed-Solomon FEC, quadrature-phase-shift-keying (QPSK) demodulation, adaptive equalization, and de-interleaver RAM into a single CMOS chip.
In 64- and 256-QAM mode, the BCM3115 enables a standard 6-MHz analog channel to carry 30 and 40 Mbps of digital data, respectively. This figure translates into a potential bandwidth with as many as 500 channels in a standard analog cable-network system.
The adaptive equalizer provides error-free transmission with a 22.5-dB S/N ratio. By using digital adaptive decision-feedback equalization, the chip can combat the effects of the channel distortions of cable-TV systems. The chip achieves transmission performance within 1 dB of theory over worst-case channels that have multipath delays greater than 1 msec and amplitudes within 2 dB of the main signal.
The chip is compatible with General Instrument's DigiCipher II technique, which is the de facto standard for North American digital cable-TV transmission and conforms to the International Telecommunications Union 64-QAM transmission standard. It comprises a 14/15-trellis code concatenated with a (128,122) Reed-Solomon code. An on-chip convolutional interleaving scheme having a depth of I=128 corrects burst errors as long as 88 msec.
The chip provides an output-data stream compatible with the 188-byte MPEG-II transport format. The measured coding gain of the concatenated code is 7 dB at an output bit-error rate of 10-10.
The QPSK receiver provides a dedicated, fixed-frequency, 2-Mbps control channel, allowing cable-service providers to individually address subscriber set boxes with multitier service options and pay-per-view programming. The chip comes in a 100-pin PQFP and costs $70 (1000). Hewlett-Packard will offer the Kayak digital set-top cable box in mid-1996.
-- by John Gallant
Broadcom Corp, Irvine, CA. (714) 450-8700.