The Smaller the Screen, The Higher the Bit-Rate

You might not have noticed that you can’t fit 20 Mb into a 10-Mb bag. Yes, you’ve guessed it. It’s time for another episode of Mobile TV Follies.

This ain’t (exactly) about the transition to DTV. Nellie, my last remaining neuron, and I have already ranted plenty about that. In a few months shy of a year (unless Congress gets cold feet), we’ll see what happens when folks relying on off-air reception discover that they can receive only low-power stations (or, if they bought NTIA-approved DTV adapters, that they can’t receive any low-power stations).


We’ll also see how off-air DTV viewers like stretched out and picture-framed video (on account of neither broadcasters nor receivers using automatic format description) and out-of-sync audio (on account of receivers not bothering to check presentation time stamps). Anyhow, that’s not what Nellie and I wanted to rant about this month.

No, what we wanted to rant about this lunar cycle was mobile TV, especially the kind that folks say TV broadcasters want to broadcast. At the grand and glorious convention of the even more grand and glorious NAB, you’ll find competing mobile-DTV transmission systems. There are A-VSB (as in advanced vestigial sideband) and MPH (as in mobile-pedestrian-handheld or miles per hour, you pick) competing with DVB-H, FLO, DMB-H and others.

That second category is for folks in the business of transmitting to mobile devices. A-VSB and MPH, on the other hand (where—hint to my secret identity—I still have five fingers), are for TV broadcasters whose main business is transmitting to TV sets.

Now, then, far be it from me to suggest that folks can’t change their minds about what business they want to be in. For instance, methinks there used to be a company called RCA that started out in the radiotelegraphy business before getting into movies, publishing, and rental cars before getting bought and then sold into itty-bitty pieces.


So, if you’re a TV broadcaster, and you decide you’d like to manufacture tap shoes or translate Sanskrit poetry or operate airport parking shuttle buses, I say, “More power to you” (and I ain’t referring to your transmitter). But, if you’re an HDTV broadcaster and think you’re going to simultaneously repurpose your programming to mobile phones via your broadcast channel, then I say, “Stop and think about what you’re doing” (and I am referring to your transmission).

This is how much bit-rate Our Beloved Commish (also known as the FCC) has allocated to each U.S. DTV channel: 19.39 Mbps. Subtract some bits for audio, PSIP and other stuff, and you’ve got something in the neighborhood of 18 Mbps for video.

This is the compression algorithm the same Beloved Commish says DTV broadcasters have to use: MPEG-2. Around a billion bits per second (1920x1080x29.97x16 or 1280x 720x59.94x16—take your pick) squeezed down to 18 million is a compression factor on the north side of 50:1. That’s a lot for MPEG-2 to deal with, especially if those original bits are something like a basketball game, but life is a compromise.

If you want to use another megabit or so for weather radar or a stock ticker or to sell as a data transmission channel for some bank or airline, you’ve got a simple business decision. Do you make more viewers happy by providing the weather or stock info or by not degrading your HDTV? Do you make more money by selling the data channel or by not degrading your HDTV? Heck, you could add one or more multicast subchannels in SD or even HD (not everything is a basketball game) if you think that’s the way to go.


The mobile TV business is something else again. Those of you born before the first NTSC color broadcast probably have a hard time making out the big numerals telling you the time on your cell phone, never mind a TV picture (and I never mind a TV picture). Even those of you just hitting sweet 16 probably don’t want to hold a tiny screen up for a two-hour movie more than once (just to say you did it). That’s why mobisodes usually last three minutes or less.

A mobisode is a mobile TV viewing episode, not a portion of a crime series. It’s a cell phone screen show, if you will (and, if you won’t, I can’t force you). Mobisodes ain’t just short; they’re also heavy on close-ups and pretty sparing of audio dynamic range. Maybe not a lot of folks would want to watch mobisodes all the time, but just 1 percent of U.S. mobile phone users paying just $1 each just once in their lives is more than enough for my retirement, thank you very much.

That’s why there are all those mobile TV transmission formats (I ain’t sure I should use the word “standards”). If a TV broadcaster wants to get hold of some spectrum and use one of them, I say, “Go for it.” The only thing you’ve got to lose is money, same as if you invest in frozen orange juice futures.


If you decide to reach those mobile folks with your DTV programming and signal, on the other hand (where I still ain’t lost or gained a digit), do yourself a favor, and work out the math. Is your main TV programming in episodes of three minutes or less? Does it comprise almost nothing but close-ups? Does it have an audio dynamic range of 6 dB or less?

I ain’t done. That was just the repurposing part. Maybe you plan on using just half a megabit’s worth of AVC compression for cell phone screens. But some mobile devices are bigger. Methinks screen sizes have already hit 7 inches. So maybe a full megabit would be better. But with A-VSB or MPH delivering 1 Mbps can mean subtracting 4 Mbps from your HDTV.

So you could lose audience, not just money.