I realize QAM is used by the cable companies. But how does this format affect HDTV picture quality and/or bandwidth? I typically hear that Comcast does not reduce the bandwidth of HD programs. Yet they have to convert the ATSC signal to QAM.

Does QAM support the full HD bandwidth? If so, then why not just rebroadcast cable using ATSC?

QAM and ATSC/8VSB are just two different transmission methods (think of them as types of modems). The compression is what affects bandwidth/quality.
So, they should be, basically, the same....bit-for-bit.

Somebody correct me if I'm wrong - but I think that the signal is encoded as MPEG 2. Presumably it's the same MPEG 2 signal whether its transmitted as QAM or 8VSB. As Ken said - bit-for-bit the same.

QAM and 8VSB are two different ways to transmit digital signals in the analog world that we live in. 8VSB was selected as a good means to transmit EM waves over the air. But cable has a higher signal to noise ratio and therefore a more efficient means than 8VSB can be used - such as 64QAM and 256QAM. In the early days there was a lot of controversy as to whether OTA should be 8VSB or DVB.

Ed

QAM, 8VSB, OFDM, QPSK, 8PSK etc. are all different ways of carrying (or modulating) a digital data stream over radio waves. They each have different advantages and disadvantages, and thus different schemes are used for cable, OTA, satellite etc.

QAM is used for digital cable in the US, 8VSB for OTA in the US (and COFDM for OTA in Europe and other parts of the world)

Because the broadcasts are digital it is possible to re-broadcast them in a number of ways. In some cases you can take the entire transport stream from one source (say 8VSB OTA) and remodulate it with few changes using a different system (say QAM digital cable. You needn't touch the mullticasting or video compression at all - though you ARE likely to have to modify the accompanying service data carrying information like broadcast frequency, logical channel number etc.

You may also decide to just carry some of the services if the original source is multicast - so you may strip out the MPEG2 video and audio streams and re-multiplex these, still not touching the video compression, so causing no quality loss.

OR you could re-compress (or rate shape which allows you to reduce the bit rate without de-compressing and re-compressing) if you want to reduce the data rate of a service OR you need to move from constant bit rate to stat mux variable bit rate, or vice versa.

So bottom line - 8VSB->QAM can be done losslessly, or it can be done with a quality change, it depends on the demands of the application...

QAM, 8VSB, OFDM, QPSK, 8PSK etc. are all different ways of carrying (or modulating) a digital data stream over radio waves. They each have different advantages and disadvantages, and thus different schemes are used for cable, OTA, satellite etc.

QAM is used for digital cable in the US, 8VSB for OTA in the US (and COFDM for OTA in Europe and other parts of the world)

Because the broadcasts are digital it is possible to re-broadcast them in a number of ways. In some cases you can take the entire transport stream from one source (say 8VSB OTA) and remodulate it with few changes using a different system (say QAM digital cable. You needn't touch the mullticasting or video compression at all - though you ARE likely to have to modify the accompanying service data carrying information like broadcast frequency, logical channel number etc.

You may also decide to just carry some of the services if the original source is multicast - so you may strip out the MPEG2 video and audio streams and re-multiplex these, still not touching the video compression, so causing no quality loss.

OR you could re-compress (or rate shape which allows you to reduce the bit rate without de-compressing and re-compressing) if you want to reduce the data rate of a service OR you need to move from constant bit rate to stat mux variable bit rate, or vice versa.

So bottom line - 8VSB->QAM can be done losslessly, or it can be done with a quality change, it depends on the demands of the application...

Are there any consumer-level products on the market that can handle these conversions? I would love to convert my QAM locals to 8VSB and send them to my HD Tivo. I have struggled for years with my OTA antennas (big building in the way), but I can pull the QAM locals from cable.

Are there any consumer-level products on the market that can handle these conversions? I would love to convert my QAM locals to 8VSB and send them to my HD Tivo. I have struggled for years with my OTA antennas (big building in the way), but I can pull the QAM locals from cable.
I doubt it - there's no market for it. Most equipment has both an ATSC and QAM tuner.

Are there any consumer-level products on the market that can handle these conversions? I would love to convert my QAM locals to 8VSB and send them to my HD Tivo. I have struggled for years with my OTA antennas (big building in the way), but I can pull the QAM locals from cable.

Yes, its called a Tivo Series 3.

Yes, its called a Tivo Series 3.

I'd be all over the S3 Tivos if I wasn't already a slave to NFL Sunday Ticket on Directv. I guess I could get another HR20 (and get my locals over the dish at MPEG4), but for some crazy reason I like DVRs that actually work more than 50% of the time. Looks like I'll need to build a new HTPC with QAM tuner.

Most equipment has both an ATSC and QAM tuner. Actually no. Many, especially entry level or the no name sets, have no QAM tuner.

Yes, its called a Tivo Series 3.
Since when does it convert from QAM to ATSC? Yes, it can receive QAM but it outputs either HDMI or component HD signals not ATSC - as far as I know.

Actually no. Many, especially entry level or the no name stes, have no QAM tuner.
OK, maybe a I should have said: Most people likely to come to AVS are likely
to buy a unit that has both an ATSC and QAM tuner. :cool:

Actually no. Many, especially entry level or the no name stes, have no QAM tuner.This is rapidly changing.

Joe Sixpack is buying NTSC/ATSC/QAM tuners in inexpensive digital TVs at wallyworld. Although only listed as having "NTSC/ATSC" tuners on the Walmart web site (http://www.walmart.com/catalog/product.do?product_id=4872540#Specifications), several of the sets I looked at in our local walmart include both ATSC & QAM tuners (assuming the user manuals were accurate).

Our local store has two 32" SDTVs with NTSC/ATSC/QAM tuners for under $300.

I'm looking at a Pioneer that doesn't have a QAM tuner.. It has most everything else. Is there an aftermarket QAM cablecard tuner avalible that anyone knows of?

Why not just rent an STB from your cable provider? ~$7 per month, full service, maintenance and warranty. Not bad for ~$84 annually. ;)

downhill,
Here is an ATSC/QAM STB (http://www.avsforum.com/avs-vb/showthread.php?p=8877015#post8877015), but it doesn't have a cablecard. If the STB Synopsis Thread (http://www.avsforum.com/avs-vb/showthread.php?t=179095) is correct, doubt you'll ever find a STB that has one.

I would look at TVs that already have the features you want.

Ratman, pricing varies greatly.
In some areas, you have to subscribe to pricier digital tiers than you want, just for the privilege of renting an HD STB. And HD STBs can be another $10 to $15.

This is rapidly changing. To the worse.
Cost cutting being job #1 (or was that Ford) dictates loosing the QAM tuner.
Samsung, Panasonic and the other manufacture (off the top of my head), for starters only have the QAM tuner on their step up models.
Your link is a perfect example. Just because it says "digital tuner" doesn't mean it has both. Especially at that price from that low end hole in the wall.

As far as STBs', there is only one new & current model that has both, but it still doesn't have a CC slot.

Ratman, pricing varies greatly.
In some areas, you have to subscribe to pricier digital tiers than you want, just for the privilege of renting an HD STB. And HD STBs can be another $10 to $15.

I know pricing varies... that's why the "~". ;)

In most of my reading, it seems that as long as your own an HDTV, most providers will rent an HD capable STB without a subscription to a digital tier of service. Also... the $15 per month rental seems more in line with a HD-DVR, not a plain ole STB.

Who knows? Anyway... it's best to call the provider and see what's available for one's particular area.

...Just because it says "digital tuner" doesn't mean it has both. Especially at that price from that low end hole in the wall.
I don't understand your comment about the TV. Although they don't list that TV as having a QAM tuner, it has one. A friend of mine's mother just bought that particular TV and it (like many TVs at Walmart and other discount stores) has a QAM tuner. She was amazed at the number of channels she got just by connecting it to basic cable.

And if you don't like wal mart, don't shop there.

Last week I got a new HDTV (Toshiba 42HL67). I also bought an antenna to pick up OTA HD. I have just the very basic/no set top box/cable package.

I just realized today that I get the local HD channels through the cable. I had been using an A/B switch to go back and forth between the cable and the antenna.

So I have the choice between 8VSB OTA signal and the 256QAM signal through cable. Will one give me a better picture than the other? My TV is telling me I have roughly equal signal strength from both, but that I have a somewhat higher signal to noise ratio through the cable (~35 compared to ~25-34 depending on channel for OTA). Actually, maybe that is more than a little difference - isn't dB in a log scale?

All in all, it would be more convenient for me to just use the cable (get rid of antenna and A/B switch, easier channel changes), but I don't trust Comcast to not have compressed or messed up the signal in some other way.


Any input would be appreciated. Thanks!

IMO, use the one that looks better and/or is more convenient for you.

And if you don't like wal mart, don't shop there. I don't!

I realize QAM is used by the cable companies. But how does this format affect HDTV picture quality and/or bandwidth? I typically hear that Comcast does not reduce the bandwidth of HD programs. Yet they have to convert the ATSC signal to QAM.

Does QAM support the full HD bandwidth? If so, then why not just rebroadcast cable using ATSC?
Cable firms (not all) use various 'rate shaping' techniques if their QAM signals are based on OTA reception (or other sources). Often fiber or other links from stations to head ends bypass OTA.

The biggest OTA change, also performed by ATSC tuner/decoders, is removing the forward error correction bits (FEC), about 11 Mbps worth, added to 19.39 Mbps ATSC signals. Cable delivery isn't subject to atmospheric interference FEC helps eliminate. Cable head ends or central sources such as Headend in the Sky also may remove program information for each source since a separate cable electronic guide is usually provided.

Rate shaping involving requantization (http://archive2.avsforum.com/avs-vb/showthread.php?p=8011489&&#post8011489), which is possible 'on the fly' with MPEG-2 without decoding, can help trim back overall cable bandwidth demands by up to 1/3, permitting more channels. But that process reduces higher resolutions/frequencies. Here's an older but still relevant Broadcast Engineering article (http://broadcastengineering.com/aps/acquisition/broadcasting_dtv_digital_cable/index.html) covering rate shaping and other cable processing. -- John