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HD 101: What is ATSC, PSIP, QAM, and 8-VSB?

HD 101: What is ATSC, PSIP, QAM, and 8-VSB?
Ben Drawbaugh
Ben Drawbaugh|May 8, 2009 11:54 AM
HD master control
Once you buy an HDTV it doesn't take long before you get confused by all the acronyms. We love us some HD, but the days of simply bringing a new set home and plugging into a coax cable are long over -- at least for most. It is pretty obvious by the latest surveys that most people don't understand exactly what it takes to enjoy HD on their HDTV, but even if you do, you may not understand what most of it means. Now while this isn't the typical make sure you have your HDTV configured to watch HD post, it will help you understand the different ways HD is delivered.

Typical for the tech industry that the term broadband isn't exactly used correctly, but it's probably more fair to say that word just change meanings as the technologies evolves. Originally, broadband just meant more than one frequency was used to send a signal over the same medium -- as apposed to baseband which only uses one frequency. The easiest example to understand is FM Radio. There is one station at 97.1 with a completely different station on 97.9. Television stations work the same way, but the frequencies are mapped to channel numbers to make things simple -- so for example, channel 8 in the US is at 180.31MHz and since each channel is 6MHz, channel 9 is at 186.31MHz. Each of these channels is like real estate -- they aren't making any more of it -- and broadcasters get a license from the FCC to use it with some non-monetary strings attached.

Packet switched vs circuit switched
Now before we get into specific broadcasting encoding methods used for over-the-air TV, cable, and satellite; lets cover something that can be a little difficult for internet geeks to understand. Unlike the internet where all the data is broken up into little packets and sent across the same shared pipe, digital TV is more like a traditional phone system. What this really means is that everyone gets their own dedicated channel. So even if channel 10 is off the air, channel 12 can't use the throughput. The two channels have no affect on each other, they each have their own discrete channels.

Encoding video vs encoding bits and modulation
Another topic that is useful to understand is the differences between encoding video vs encoding bits. Again being internet savvy you're probably familiar with various codecs like MPEG-2 and H.264, but this isn't the only kind of encoding. In fact encoding just means transforming information into different formats, but more importantly, in order to send these bits across a wire or through the air, they must be modulated. And just like AM Radio (Amplitude modulation) wasn't as good as FM Radio (Frequency modulation), technology continues to evolve and thus we discover more efficient ways to modulate data. So in other words as technology evolves we find new ways to send more bits across the same 6MHz channel.
MPEG Encoder
MPEG-2 encoders

Here is the US -- and in some other countries -- we've used NTSC for about 50 years to send over-the-air TV to homes. The so called digital transition is about transitioning us from NTSC to ATSC. While understanding the ins and outs of either standard is well beyond the scope of any 101 article, you should know that both use the same size channels (6MHz) and that until a channel goes all digital, it is actually broadcasting on both frequencies (simulcasting).

To ensure that viewers can find the right channel while the broadcasters move around, a technology called terrestrial virtual channel table (TVCT) was included into the Program and System Information Protocol (PSIP). This enabled the broadcasters to send specific data like programming information, but more importantly it supports channel mapping information. The key here is to realize that when you do a channel scan your TV will automatically know that when it discovers channel 24, that you expect to find this station at channel 10. As the digital transition is completed and broadcasters move frequencies -- either away from VHF so other services can use it, or are simply moving back to their original channel assignment -- most ATSC tuners will require users to rescan in order to update the table.

PSIP Generator
This equipment generates PSIP information.

The other highlights of ATSC that are useful to understand is that it uses 8VSB modulation -- this fact and $3 will get you a cup of coffee -- and that each 6MHz channel has a dedicated 19.39 Mbps of throughput. The ATSC spec has lots of good stuff in it, but for us HD fans the important parts are that it supports MPEG-2 encoded video and AC3 encoded audio -- a multiplexer that combines the audio and video is pictured below. Interestingly, 1080p30 is actually a part of the spec, but most don't expect it to ever be utilized because 19.39Mbps isn't really enough to send artifact free MPEG-2 HD at 1080p -- or 1080i for that matter when it comes to sports.

ATSC multiplexer
MPEG2 and AC3 multiplexer.

This is a term thrown around a lot these days because it's actually pretty useful to understand. Basically it is the type of modulation that most cable providers in the US use -- including Verizon FiOS. The coolest thing about QAM is that since it doesn't need all the error correction of an over-the-air modulation technique, it is much more efficient. This efficiency pays out by delivering 38 Mbps of throughput per 6Mhz channel -- about twice what ATSC is capable of. The real problem with QAM -- no fault of it's own though -- is that when a cable operator uses QAM, it can also inexpensively encrypt the signal, which is why you might have heard about clear QAM. All this really means is that the QAM signal is not encrypted, which means all you have to do is plug the coax into your tuner and start watching HD. There is actually some good HD on clear QAM too because luckily the FCC doesn't allow operators to encrypt channels that can be received over-the-air. The bad news is that there isn't any guarantee that your TV will be able to find all the channels. Well, it isn't that it can't find them as much as it is the fact that some providers strip out the PSIP data when shaving bits out -- more on that later. Some devices that support clear QAM allow you to manually map the channels, but most -- like the TiVo HD -- don't have this feature. The cable provider's STBs don't have the channel mapping problem because they have access to a virtual channel table. The problem is that unless you have a CableCARD, you don't have access to the table.

QAM Modulator
QAM Modulator

With the advent of HD bandwidth has become a problem for all content providers, but it helps if you understand how it is managed. Expanding a cable plant's QAM infrastructure can be very expensive, especially when you move closer to consumer's homes because you need more equipment. And of course QAM doesn't evolve overnight, so operators have to find new ways to make more efficient use of the bandwidth currently available. One way they do this is by putting multiple HD streams on one channel. This isn't really a problem when you take two 19.39 Mbps ATSC streams and modulate them on one 38Mbps channel, and truthfully 80% of the time all the bandwidth isn't used -- some 1080i programming uses as little as 10Mbps. So what operators do is put three 1080i channels per QAM channel, which doesn't take 11th grade math to figure out there isn't always going to be enough bits to go around -- bet you wish this was packet switched now. So the operators deploy a technology called statistical multiplexing to try to dynamically allocate enough throughput to each stream. The theory is that none of the streams will need all the bandwidth at the same time. Unfortunately in practice this isn't the case, and our beautiful HD turns into block city.

The more recent technology that operators have turned to is switched digital video (SDV). This is where instead of sending every channel to every house, all the time, only the channels that are being watched are being sent. This is actually pretty cool technology but the problem is there is the off chance that a channel won't be available when you want to watch it -- very rare -- and even more common, your one-way CableCARD enabled Media Center or TiVo won't be able to tune in the channels at all. Recently operators have been giving out a "tuning adapter" which enables one-way CableCARD host devices to work with SDV, but this is all pretty new and Microsoft has yet to even support it -- although we fully expect support in Windows 7.

SDV Diagram

The good news is that while technology continues to change, due to compatibility reasons these technologies evolve slowly. So now that you understand most of this -- hopefully -- it should serve you well for years. The one thing we didn't touch on though was satellite technology and mostly this is because both of the providers in the US require you to use their STBs anyways, and there really aren't many reasons you'd need to know what types of modulation they used -- at least not in the context of this port.

Got a basic HD topic that you'd love for us to explain? Tired of Google's complicated answers when you asked for a simple explanation? Hit us up at ask at engadgethd dawt com and keep an eye on this space -- your topic could be next.