On 29/01/2011 04:33, Charles Craft wrote:
> If you can get a tuner like
> (http://www.hauppauge.com/site/products/data_hvr950q.html) for<  $100
> why can't someone like Ramsey come out with a consumer grade modulator
> for ATSC.
Well MPEG2 and MPEG4 are designed that decoding should be easier than=20
encoding

"tuners" in PCs only to Analogue with a ADC+ basic DSP chip to=20
demodulate the signal. Then the PC software sorts out the decoding. Only=20
the very highest end PC can do re-time encoding to MPEG2 (and esp MPEG4)=20
with any kind of sensible quality and bit rate.

There is little market for Analogue to Digital RF consumer system.
A consumer system would need chips derived from maybe a Camcorder.
The camcorder chip + video ADC chip would convert Analogue video to MPEG4.
Then you need something to make that into a Transport stream. No=20
consumer chips.
The transport stream then goes to a "modulator". Which system? China,=20
Europe, North America, Japan and South America are completely=20
incompatible digital systems. no Consumer ICs exists for that. Expensive=20
FPGAs and ASICs are used because there is no volume market.

Ramsey stuff is mostly obsolete stuff. They don't develop state of the=20
art systems.

analog video->-[ ADC ]->-[MPEG Encoder]->-[Transport Stream=20
Mux]->-[Viterbi/FEC]->-[Modulator]->-[UHF]-

The Modulator is country specific unlike PAL/SECAM/NTSC where the same=20
modulator will work apart from sound
For PAL/SECAM/NTSC the source unless RGB or Component already has the=20
Colour encoding. The frequency of the audio subcarrier simply needs=20
changed per country. The Modulator is just AM. They don't bother=20
filtering the lower side band on domestic modulators. So Analog RF=20
modulator is very simple and could even be built on stripboard with two=20
transistors (4.5 to 65.MHz FM osc for sound and a AM osc at about 500MHz=20
tuned with a brass slug).

A PC Digital  tuner card simply is a  Zero IF receiver with I & Q=20
analogue outputs. This goes into a single chip DSP (unusable as a=20
Modulator) which has two ADCs running at about 50MHz sample rate (maybe=20
100MHz). The chip  is specific usually for Country System  as ATSC,=20
ISDB, DVB-T, DVB-T2 and DMB are radically different. Usually QAM is a=20
different design of chip (cable TV and cable Modems) and simpler. DVB-S=20
and DVB-S2 need different chips too. Modern chips have the Viberbi=20
decoder/FEC error correction system to recover the MPEG2-TS. The output=20
of the chip is either an entire transport stream, or some chips (used in=20
USB DTT sticks) can demultiplex and remultiplex the serial data to have=20
just the video, data and audio streams needed for selected channel. The=20
rest of the "work" is done by software codec on PC. A setbox or TV set=20
uses a DSP chip. If there is a CAM, it goes between the Tuner DSP chip=20
and Video/Audio DSP. Some setbox/TV chip have both DSP (demodulator and=20
Codecs) in same chip.

You might as well ask why there never has been a consumer Nicam encoder=20
for Digital Stereo on Analogue PAL RF modulators. Only ZweiKanelton=20
Analogue Modulators ever had "Stereo" as those simply use two FM=20
subcarriers. All other TV modulators are mono as the cost of a Nicam=20
encoder is prohibitive.




--=20
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