03.04.2003 12:00 PM
Originally featured on BroadcastEngineering.com
ATSC receivers via software
Single HD frame of NBC's Law and Order captured, demoded and decoded via software.
More than 100 companies have now banded together to form the Software Defined Radio (SDR) forum. This group is pushing for a simple generic hardware platform whose operation would be defined by software. The proposed system would allow items such as modulation techniques, bandwidth, operation and function to be defined by the software that is loaded into the device.
Software controlled RF devices may allow generic hardware to perform many different functions. Such examples are devices such as the TiVo, which are basically Linux-based PCs performing as home receiver and recording devices.
Another group pursuing software based RF reception techniques is GNU. The GNU project was launched in 1984 to develop a Unix like operating system called GNU, which is a recursive acronym meaning "GNU Not Unix." GNU is considered "freeware." The Free Software Foundation is the principal sponsor of the GNU project. The code generated by the project is a collaborative effort of volunteers hoping to offer alternative software. While the code can be shared and modified, developers using the base code cannot create proprietary versions of it. There are variants of GNU that use the Linux kernal instead of the original Unix-based one.
A sub-group of the overall GNU effort is GNU Radio. It has developed a number of projects including a FFT display of high speed ADC RF input. It also developed a single channel FM demodulator in which input from the IF of a low-cost AM/FM radio kit is fed into high speed A/D and then to a software program for demodulation and detection. It also boasts of dual channel FM demodulation. The RF input is from a cable modem tuner module. The tuner module IF output at 5.75 MHz is connected directly into the 20 million sample/sec A/D to extract and demodulate two stations simultaneously.
But the most interesting project to date from the group is an ATSC receiver implemented via software. Single frame snapshots were captured February 15 from KSBW-DT, ch 10 Salinas, Calif. Samples from NBC's Law and Order were recorded to disk in real- time at 20M samples/second using GNU Radio project software and A/D card driver. Once the samples were on disk, they were processed using a GNU Radio ATSC receiver program. The output of the ATSC receiver software is an MPEG transport stream consisting of a sequence of 188-byte packets. The MPEG transport stream was then fed into a Xine MPEG (software) player and displayed. The 2730 x 1088 display files were generated from within the Xine player.
The group plans to push the hardware to software handoff as close to the antenna as possible. Between the efforts of the SDR forum and the GNU, truly modulation agnostic receivers might someday finally put the 8-VSB and COFDM (or any other modulation scheme lurking) arguments to rest.
For more information visit www.gnu.org.
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