April 21, 2010 5:48 PM   Subscribe

Digitial TV - why does it lose reception in distinct cycles?

We've got pretty poor DTV reception, we're behind a hill, and use an indoor antenna. In particular, for one station group, we often get poor reception when we're walking around certain parts of our house - the kitchen, for example. So the rule is; stay on the couch if you want to watch.

However, the way the reception breaks up is weird. The audio disappears in cycles with a frequency of approximately one second. So we get half a second of perfectly clear sound, then half a second of silence, then half a second of sound, then half a second of silence. Why would it happen in this way?
posted by Jimbob to Technology (8 answers total)
Best answer: Warning: simplified, but still lengthy, technical explanation follows.

Old-style analog television relied on an analog signal. Imagine just the audio for a moment (it's much simpler). You've seen audio waveforms, right? At each moment of time, there's an amplitude. This amplitude roughly corresponds to how much sound pressure there is, or how much voltage to apply to the speaker. So, if you interrupt an analog audio signal, and then uninterrupt it, you immediately pick back up and go on with the sound.

Furthermore, if you distort the radio signal transmitting the audio, the audio you hear will be distorted. Add some static to the signal, and you hear the static on your end. Amplify the signal, your audio gets louder. There's a direct one-to-one relationship between the transmitted signal and the sound your speaker produces. There was no way to know that the audio was wrong, and with poor reception, the sound simply had a bunch of static or pops in it.

But digital doesn't work this way at all. At its absolute base level, it does approximate an analog signal by remembering the amplitude frequently enough to sound good to humans. So, instead of an absolutely smooth wave, it looks more like a staircase. But, get enough steps on that staircase and it's indistinguishable from a smooth waveform. This is the fundamental idea behind digital audio.

However, digital TV doesn't transmit a raw digitized waveform. Instead, it compresses the signal. Essentially, it applies an algorithm that reduces the total amount of information that needs to be transmitted. Then, the opposite algorithm is applied, and the digital waveform is reconstructed on your end. (The exact details of the compression scheme used are irrelevant.)

In order to achieve this compression, the waveform is broken down into blocks. The compression algorithm is run on the whole block, redundant information is discarded, and the compressed block is transmitted.

But, if any part of the transmission of that block is bad, due to radio interference or whatever, then the entire block cannot be reconstructed. It's like screwing up just one or two clues on a crossword puzzle... get them wrong, and the entire puzzle is completely unsolvable. There is no "close enough" in digital transmission. Either the bits got through unscathed, or they're wrong and must be thrown away.

It so happens that with digital TV, the length of the block is a fraction of a second. So, if you're getting spotty reception, an entire block may make it through unscathed, and so it plays back that moment of audio. Since the block got through, the audio is perfect. But, if even just a little of the block is screwed up, that moment of audio will not be played back. (If you were to play it back, it would not sound anything at all like the soundtrack to your show. It'd probably just sound like full static or speaker-breaking-popping, so they choose not to even bother.)
posted by Netzapper at 6:33 PM on April 21, 2010 [2 favorites]

Best answer: You are experiencing Fast Fading as a result of multi-path interference. You receive a direct line of sight signal directly from the broadcast tower, but you also receive a reflected version of that signal at the same time, likely either bouncing off the mountain, or diffracting over the top it is. The reflected signal is delayed, so when it combines with the desired signal at the antenna it causes destructive interference.

Since both signals are sinusoidal, their interference patterns are also sinusoidal, so your sound goes out in cycles, as the interference temporarily pushes the Signal to Noise Ratio at your receiver below the threshold, causing it to cut off.

You would likely benefit from a small outdoor antenna, or a directional antenna if most of your channels come from the same direction.
posted by I_am_jesus at 6:43 PM on April 21, 2010


ATSC transmission is not packer based standard. It uses Reed Solomon block coding as an outer encoding scheme, an interleaver and convolution trellis coding as an inner encoder. It is not such a simple matter of messing up a few bits, it is a matter of pushing the received bit error rate past the point that the combined efforts of the channel codes can correct errors. ATSC transmission can operate at a bit over 5% symbol error rate, but if you drop just a bit below that point, you are correct that you will lose all of the data. With convolutional coding it is an all or nothing proposition.
posted by I_am_jesus at 7:04 PM on April 21, 2010 [2 favorites]

Response by poster: Thanks, looks like it's time for a rooftop antenna. You could have skipped the first 5 paragraphs of your answer, Netzapper, but no bad. I always figured DVT, unlike analogue, would be either 100% perfect, or nothing at all.
posted by Jimbob at 7:07 PM on April 21, 2010

Just to note Jimbob, that when they turn analogue off in the next year or so (not sure what the timetable for tassie is), they'll ramp up the signal strength, it is currently restricted so as not to interfere with analogue broadcast.

This may or may not be a fact.
posted by wilful at 11:18 PM on April 21, 2010

Response by poster: when they turn analogue off in the next year or so (not sure what the timetable for tassie is)

It's still 1987 here, wilful.
posted by Jimbob at 1:39 AM on April 22, 2010

I read your question last night and I've been mulling over it for a while. I was going to advance the theory that I_am_jesus posted above; that what you're seeing is a form of multipath, and the signal is fading in and out (the 1s in-out cycle) due to the two received signals alternately reinforcing and canceling each other.

ATSC — the new digital broadcast standard — has a reputation of having significant issues with multipath, so it at least sounds plausible to me. I've never seen ATSC cut in and out on such a long interval when it's just purely a weak-signal issue; the behavior I've seen in that case is video blocking and fast audio dropouts that just increase until the signal is unwatchable, followed by the receiver failing to lock ("no signal"). Of course, ATSC hasn't been around all that long and my experience with it is limited compared to good old NTSC.

I'd try moving your receiving antenna around and see if that improves things at all. (I assume you've already tried moving the antenna aerials around, pointing it in various directions, etc.) If that doesn't do the trick, I'd probably say that the guaranteed fix is an outdoor antenna, something optimized to be highly directional rather than just having lots of "metal in the air," on a rotator. The combination of being outside, up higher, and being able to focus the receive pattern on the line-of-sight path ought to fix it.

Incidentally, the same fix would also apply if I'm just totally wrong and it actually turns out that Netzapper is right, although if you were sure it wasn't multipath you might want to go for an omni rather than a directional antenna up top.
posted by Kadin2048 at 9:35 AM on April 22, 2010

I just realized you're in Australia, which uses DVB-T rather than ATSC. That might undermine my theory a bit, because DVB-T is supposed to be better at handling multipath than ATSC, plus my experience with ATSC in weak-signal situations doesn't really apply.

I still think it's more plausible, but I'm less confident than I was when I was making the assumption you were in the US.
posted by Kadin2048 at 9:37 AM on April 22, 2010

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