Please help me understand something

mike81

I am hoping that someone can help me with this question. I know that in order to get HD on your HDTV, you need to connect the HD box to the TV with HDMI cable. It's my understanding that the 1080p can not be transmitted via coaxial cable. If that is so, then how does the 1080p signal get from the dish or cable company to your HD box when it is traveling through coaxial? Is it that the 1080p can be transmitted over coaxial as long as it is a certain type like RG-6? Am I missing something here? If there is already an explanation somewhere, could someone give me a link to it? Thanks for the help.

BIslander

The signal from a satellite dish or a cable company is not video. The set top box or tuner in the TV turns the signal into video and at that point HDMI or component cabling is needed.

m_vanmeter

whether the HD video/audio signal is sent via satellite, cable, or over-the-air from your local network broadcaster, it is highly compressed. The function of the set top boxes or the built in ATSC/QAM tuner (for over-the-air and some cable channels) is to decompress and decode the signal, expanding the amount of information considerably in the process. That is when you need the HDMI cable for "digital HD" or the three component (red, green, blue) cables for "analog HD".

mike81

Thanks very much for the replies. I understand now. So does it matter whether or not the cable going into the box from the dish or cable company is RG-6? Will RG-59 cause a poorer picture?

m_vanmeter

for short runs, no. the advantage of RG-6 over RG-59 is lower losses over longer lengths of cable.

Scottnot

I don't think that's 100% or even 50% accurate.

Actually, the primary function is to convert the wide-band RF signal to either a digital signal (HDMI) or an analog (compontent signal).
The ATSC/QAM tuners primar function are as "tuners", they receive a multitude of wide-band RF signals, "tune" to one of them and then convert to drive the video display.

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Charles Krug, 1978 Vintage Selection, Napa Valley Cabernet Sauvignon
Beringer, 1980, Napa Valley Cabernet Sauvignon Port
Morano, 1980 Vintage Port
Beaulieu Vinyards, 1982 Napa Valley Cabernet Sauvignon
Stratford, 1983 Merlot
Chateau La France, 1986 St. Emilion
Sutter Home, 1982 Napa Valley Zinfandel

The lonesome sipper

mike81

So what would be accurate?

Scottnot

In the context of this thread, mention of compression is sort of "off topic" as it's not directly relevant to the issue at hand.
The use of the phrase "highly compressed" can be quite misleading as well and seems to carry a negative connotation.

Indeed, digital TV broadcasting makes use of a compression and encoding scheme known as MPEG-2. This encoding scheme allows for the compression of images to "fit" into a reasonable amount of bandwidth. The use of MPEG-2 enables the reduction of the amount of data by about 55 to 1.

MPEG-2 is the industry standard for broadcast TV, DVDs and several satellite TV broadcast systems. It is very efficient at discarding image detail that the human eye tends to ignore anyway. The image quality is far better than traditional analog TV.

Once again, all "off topic" as regards your OP . . . but, you asked.

__________________
------------------------------------------------------------------------------------------------------
Charles Krug, 1978 Vintage Selection, Napa Valley Cabernet Sauvignon
Beringer, 1980, Napa Valley Cabernet Sauvignon Port
Morano, 1980 Vintage Port
Beaulieu Vinyards, 1982 Napa Valley Cabernet Sauvignon
Stratford, 1983 Merlot
Chateau La France, 1986 St. Emilion
Sutter Home, 1982 Napa Valley Zinfandel

The lonesome sipper

m_vanmeter

OK, to avoid a semantics war, I withdraw the flash point word "highly"....but "compressed" remains.

I also submit that "decode" and "convert" mean the same thing in this context.

So, are we up to maybe 75% accurate now.

DTV_Engineer

No, actually he is quite accurate indeed. If you look at a block diagram of a DTV receiver, you will see these elements:

1) A tuner, which takes RF modulated as either 8VSB or QAM, and converts it to a packetized ASI stream. For ATSC signals, this is a 19.39Mb/s signal. The tuner also removes the pseudo-randomizing code that is added to the data prior to transmission to prevent certain data patterns from producing low modulation of the RF carrier.

2) A PSIP decoder, which finds the various tables in the ASI stream and decodes them so as to know where to find audio, video and metadata, and how they are encoded

3) An MPEG-2 decoder which takes the video packets for the selected stream, and converts them to a continuous serial HD data stream at 1.485Gb/s for HD or 270Mb/s for SD

4) A frame buffer which aggregates the video data into a coherent video frame so it can be presented to the display device (LCD, plasma, or whatever). Scaling is generally also performed at this point.

5) A Dolby AC-3 decoder takes the audio packets for the selected stream and decodes them to a flow of continuous AES-EBU audio data, which is then converted to analog.

6) Various blocks for metadata functions like closed captioning and electronic program guide read other packets or pieces of the video frame buffer, and produce relevant displays that get summed back into the frame buffer for display.

7) The final output section, which is the video display itself, and the audio driver, power amp (if any) and speakers (if any).


Clearly, there is far more going on than mere demodulation, which is what a mere tuner does. A DTV receiver is taking an RF carrier that is less than 6 megahertz wide, transforms it to the raw compressed data at 19.39 megabits per second, and then uncompresses that to its original uncompressed 1485 megabits per second.


Getting back on topic, yes -- not only is it possible to connect 1080p video over coaxial cable, that's precisely how we do it at the studio. Belden 1694 is the most commonly used cable, and is terminated with 75-ohm BNC plugs. As a rule, you can send this 3G HD-SDI video several hundred feet before it needs to be re-clocked.

The reason HDMI exists is to be a more consumer friendly format that has a degree of compatibility with DVI. Generally speaking, HDMI cables don't hold to the same specifications for spectrum flatness that you find on precision coaxial cable... but for short cable lengths, that isn't a big problem.

-- Jeff

Scottnot

As m vanmeter already said, what's the point of getting into a semantics war . . .
in this case what do the words "primary" and "tuner" really mean?

And frankly, I don't think the OP gives a rat's ass about "packetized ASI streams", "PSIP decoders", "data stream bit rates", "frame buffers", "metadata functions", and other such techno-garble.

__________________
------------------------------------------------------------------------------------------------------
Charles Krug, 1978 Vintage Selection, Napa Valley Cabernet Sauvignon
Beringer, 1980, Napa Valley Cabernet Sauvignon Port
Morano, 1980 Vintage Port
Beaulieu Vinyards, 1982 Napa Valley Cabernet Sauvignon
Stratford, 1983 Merlot
Chateau La France, 1986 St. Emilion
Sutter Home, 1982 Napa Valley Zinfandel

The lonesome sipper