DQT1000 Digital to QAM Transcoder Advanced Applications

The introduction of the DRAKE DQT1000 represents the addition of yet another unique product to DRAKE's growing line of products for the reception and processing of digital television signals. The DQT1000 is unsurpassed in combining multiple functionality in one single unit of rack space. It contains two tuner/demods with 8VSB and QAM capabilities, MPEG program filtering, a transport stream multiplexer, a QAM modulator and a low noise high output power agile upconverter. In addition it may be special ordered with a single tuner/demod and a field installable ASI Input Module is available. The DQT1000 can be remotely controlled and programmed via its RS232 serial interface. The DQT1000 is capable of transcoding several different digital input combinations into a QAM output on any desired broadcast or cable channel for insertion into a cable system channel map. A program filtering function allows inclusion or deletion of any individual program in the digital transport stream.

The following applications detail the possible uses for this truly agile product.

Off-Air/8VSB to QAM Transcoding The DQT1000 can transcode a single off-air/8VSB digital broadcast signal to a QAM output. The demodulated 8VSB broadcast signal consists of a MPEG2 transport stream (TS) of up to 19.4 Mbits/s. With the appropriate Forward Error Correction (FEC), the resulting TS with this additional "overhead" will still "fit" into the selected 6 MHz output channel using 64 QAM modulation. Null Packets will be added as required. Multiplexing Two Off-Air/8VSB Broadcast Signals into a 256 QAM Channel This is probably the most common use for the DQT1000. The standard DQT1000 is supplied with dual demodulators and will accept two off-air inputs. These can be sourced from a single off-air antenna and a splitter or from two separate dedicated off-air antennas. The demodulated transport streams (up to 19.4 Mbits/s each) are multiplexed resulting in a transport stream with a data rate as high as 38.8 Mbits/s. After the addition of the required FEC, the resulting transport stream will still "fit" in the selected 6 MHz output channel using 256 QAM. The DQT1000 may be special ordered with a single demodulator if desired.

Multiplexing an Off-Air/8VSB Broadcast Signal with an ASI Source into a 256 QAM Channel With the addition of the optional ASI Input Module, the demodulated transport stream from the built-in tuner /demod can be multiplexed with the transport stream supplied to the ASI Input Module. This ASI source could be a file server, a standalone demodulator, a fiber link from a broadcast station, many satellite IRDs, or, in general, any source supplying an ATSC based transport stream not exceeding 19.4 Mbits/s. As in the previous example, if the combined transport stream does not exceed approximately 38.8 Mbits/s, it will still appropriately occupy the selected 6 MHz output channel after the addition of FEC using 256 QAM modulation.

Program Filtering A "select program" function is provided to allow the operator to select which of the MPEG programs present on the A and B input streams to be included in the output multiplex.

Regenerating a QAM Signal from a Noisy Source Since the DQT1000 demodulates the input signal (QAM in this case) the resulting MPEG2 transport stream ("digital baseband" signal) is identical to the input channel. It error corrects, then re-modulates the selected output channel with this error corrected transport stream with the desired QAM modulation. Thus, it can be used to "regenerate" a "clean" QAM signal on any output channel.

QAM Channel Translation Typically referred to as channel processing in the cable industry, the DQT1000 can input a QAM signal and effectively "translate" it to a different channel on the output. However, unlike most analog signal channel processors, this is not a heterodyne process. While a hetrodyne processor is excellent for analog signals, for digital signals it is best to use a demod/remod process making full use of FEC (Forward Error Correction). This is done in the DQT1000.