Last year, I moved. For many reasons, I did not subscribe to cable television. To save money and avoid wasting time watching bad TV, I decided to see how much I missed it. I purchased a $10 set-top antenna, positioned it as best I could, and waited to see if I suffered withdrawal symptoms from not having cable or a satellite subscription. Aside from missing The Daily Show, I was fine.

At first, I was only watching regular old analog TV. I live in San Francisco, and my home is only a few miles from Sutro Tower, the main TV transmitter for the San Francisco Bay Area. Although I'm only a few miles away, at least two hills block a direct line of sight to the tower, and my home is located in a valley of sorts. My regular reception was full of ghosts.

Some ghosts are worse than others. The very high frequency (VHF) stations, channels 2-13, come in quite well. Some channels have faint transparent ghosts, but it is almost imperceptible. The ultra high frequency (UHF) stations, channels 14-69, are noticeably worse because, with the exception of one channel, I lacked a direct line of sight. Digital TV reception became an attractive idea because the stream of bits can self-correct. If you have enough signal to decode, you get a perfect picture. The flip side is that if you don't have a signal, you get a blank screen.

As is often the case, external events intervened. KCSM, one of the PBS stations in the San Francisco area, lost the lease on their analog transmitter site in May. As required by the FCC, KCSM had been broadcasting a digital signal on channel 43 in addition to the analog signal on channel 60. When the analog signal went off the air, I decided to buy the equipment I needed to receive their digital signal. As a bonus, most of the other stations in the area were already broadcasting digitally, so I would receive clear pictures on all of the channels I watch.

Tinkering with electronics can become a great time sink, and this project certainly had the potential to develop into a multi-thousand-dollar sinkhole. To hold down costs, I decided to avoid upgrading my A/V system to high definition (HD). The cost of a TV upgrade would be tolerable, but I can no longer imagine living without my TiVo, and high-definition TiVos are still quite expensive. Upgrading everything to high definition is a much more complex project because I would want to get a DVR of some sort, possibly a media PC built around a card from pcHDTV.

Introduction to Terrestrial Digital TV

There are many types of digital television broadcasting. My project was to receive terrestrial digital signals. The word terrestrial is used to refer to traditional land-based antenna broadcasts, as opposed to cable or satellite television. Terrestrial digital TV broadcasts have a whole new set of standards, defined by the Advanced Television Systems Committee (ATSC). Naturally, to receive digital signals, you need an ATSC (or, often, simply "digital") tuner. To get an ATSC tuner, you can purchase a set-top box with a tuner, or an "integrated" HDTV set that has a tuner built in. I decided to go with a set-top box because I wanted to keep all my other TV gear, and I wasn't quite ready to take the high-definition plunge.

Digital TV is more than clear pictures at standard definition. A standard analog TV channel requires a 6MHz bandwidth, which is a vast amount of spectrum for the data it carries. By chopping up the TV signal and transporting it digitally, the 6MHz bandwidth can be converted into a bit stream of approximately 19Mbps. That bit stream can be used to carry anything, though the most common use is to carry MPEG-2 video streams and associated audio. 19Mbps is enough capacity to carry multiple TV broadcasts; many stations use this capability to broadcast multiple "subchannels" within the assigned digital channel.

For example, KCSM's first subchannel, 43-1, broadcasts the TV signal; the second subchannel, 43-2, is a simulcast of their excellent jazz radio station. As it turns out, the radio station does not come in too well at the house, so converting to digital TV would also bring the radio station in. Subchannels may be unrelated content as well; Mark Cuban struck a deal with a Bay Area local affiliate to carry HDnet broadcasts on channel 4-2.

Many analog TV stations were given a second channel to broadcast digitally. (See the National Association of Broadcaster's channel map.) As an example, the Fox network affiliate in San Francisco was on channel 2 with their analog signal. Their digital TV broadcast is on channel 56. Rather than force TV viewers to remember that we need to go to channel 56 for the high-definition digital broadcast, though, a digital TV facility called the Program and System Information Protocol (PSIP) can "remap" channel numbers. Fox's Bay Area digital channel is broadcast in the spectrum for analog channel 56, but the PSIP data transmitted with the signal remaps the number to channel 2-1.

Staying in Thrall to the TiVo

I have now owned a TiVo for approximately a year and a half, and I will not give it up. Standalone TiVo units are standard-definition devices, so I needed a set-top box to downconvert high-definition signals for them to be suitable for the TiVo and my existing standard-definition television set. A set-top box might also come in handy later, since many high-definition sets do not include a digital tuner, although recent FCC rules requiring digital tuners are beginning to take effect for larger sets. With a digital tuner, I could feed my TiVo flawless standard-definition signals to record, but recording digital TV off the air is not (yet?) a typical use case for the TiVo. There were three basic challenges I faced:

  1. Select a set-top box that can provide multiple video outputs simultaneously. If I ever wanted to watch high-definition TV while recording a standard-definition version, or even just left the set-top box in high-definition output mode when I was away from home, I would want to ensure that my TiVo received a standard-definition signal and could record.
  2. Find a set-top box that can be controlled by the TiVo, so that it can select the appropriate channel to record. This means that TiVo must send the command to change to channel 5, and the set-top box will select channel 5.
  3. Obtaining guide data for the digital channels. While a TiVo can be used as a VCR, it is much easier to use when you have guide data. Guide data does not exist for the second or higher subchannel.

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Selecting and Connecting the Set-Top Box

To receive digital signals, the TiVo needs a hand. External HD receivers will decode digital signals, and they can all downconvert a high-definition signal into standard definition for non-HD equipment. That much is straightforward, and can be served by almost any set-top box on the market. New HD receivers can be purchased for less than $300, but as is often the case, there were two additional features I wanted that are not common at the entry level.

First was the ability to output the video to multiple output ports, as well as simultaneous output of high-definition and standard-definition signals. If I were to upgrade to a high-definition set, I would want to watch TV in high definition, but I might still want to simultaneously record that program in standard definition on my TiVo. This is a common feature, but not well documented in product manuals.

The second feature is tied into the guide data. Digital channel numbers consist of a main channel number and a subchannel identifier, usually written with either a dash (2-1) or a dot (9.1). TiVo guide data does not include programming on all of the digital subchannels. In most cases, this isn't a problem, because there is only one subchannel, and it is an HD simulcast of the analog broadcast. However, TiVo's channel changing is not subchannel-aware. To use a TiVo to control a set-top box, the set-top box should automatically fill in a subchannel identifier of "-1". If the TiVo changes the channel to 2, the set-top box needs to interpret the request as a digital tuner request, and fill in the subchannel to change to "2-1". Again, many set-top boxes will do this, but not all will, and this behavior is also poorly documented in product manuals.

The third, and most important, feature is to get a set-top box that can be controlled by a TiVo. Set-top boxes can be controlled in two ways: either by an infrared emitter that emulates a remote control, or with a serial port. I had a preference for the serial port because of previous experience with somewhat flaky infrared devices.

With a little bit of investigation, I learned that the TiVo service does not really support terrestrial digital TV receivers. The TiVo database of set-top devices consists mainly of satellite receivers, while high-definition tuners are generally not supported. Many satellite receivers also include high-definition tuners, so I decided to look at the TiVo cable and satellite box compatibility list.

I began by searching the compatibility list for a receiver that could be controlled by the serial port, and started to narrow down the list. Eventually, by reading reviews, I learned that the LG LSS-3200A, Sony SAT-HD300, and Hughes HTL-HD offered everything I wanted. (All three units have identical hardware, manufactured by LG, though the prices vary.)

TiVos can select channels using the serial port, and will also provide simultaneous outputs and fill in a default subchannel identifier of "-1". There are tuners that are said to be better, but they lack the ability to be controlled by a TiVo. I purchased an HTL-HD from eBay, in large part because the auction price was much lower than its nearly identical LG and Sony twins. By running the setup program on it, I was able to configure it as a digital tuner without DirecTV service. The setup program also includes a channel scan that will identify both analog and digital signals.

The main downside to the HTL-HD is that it does not have a good way to select which channels are in use. If you (or a TiVo) repeatedly changes the channel to the same number, it will cycle through its inputs. If the number "4" is entered repeatedly, it can have frustrating results as the HTL-HD cycles through its inputs. The first time, it will go to the digital broadcast on 4-1, automatically filling in the subchannel. However, a second channel change will cause it to go to analog channel 4, even if it is marked as a channel that is not received. Even worse, a third consecutive request to go to channel 4 will change to analog cable channel 4, which is just snow in my setup, since I am not a cable subscriber.

Although I have no analog cable channels configured, the HTL-HD will accept requests to change to it because there is no way to remove channels from direct channel access requests. In the end, this is a minor frustration that requires manual intervention on my part. Because the first tuning request will attempt to find the digital channel, I will look at the To Do list once a week. If there are two recordings that will be on the same digital channel, I insert a five-minute manual "junk" recording on another channel between my two scheduled recordings. When the second program rolls around, it attempts to change from the junk channel to the target channel, and pulls in the digital channel again.

Once I obtained the set-top box, I connected it to my TiVo as a second input. A cheap set-top antenna was already used to receive over-the-air broadcasts. I connected the HTL-HD and its antenna to the TiVo's video inputs as a second input source, with the TiVo serial control wired to the HTL-HD for channel changing purposes.

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