iXBT Labs - Computer Hardware in Detail






External PC TV Tuners

April 21, 2003

Today I start a series of reviews of external PC consoles which are meant for TV broadcast reception and displaying TV channels on a PC monitor. First of all, let's touch on a bit of history and theory to understand how they work.

A classical TV tuner is no wonder today. Many PC users have them in their computers. There are two most popular types TV tuners installed in PCs: a separate card for a PCI bus, and a tuner integrated into a graphics card. The advantages of such solutions are obvious: the TV window is formed on a software level and can be easily resized, moved around a PC screen, minimized, fitted to full screen or set as wall paper. In some versions you can even adjust its transparency. As a rule, all such devices have developed software able to record clips or separate frames into files, compress with MPEG and MJPEG algorithms with the CPU, make records according to a timetable etc. However, there is a reverse of the medal: the radio-frequency module is located inside the PC case, which tells upon its sensitivity, selectivity and, therefore, image quality. The PCI bus is heavily loaded when such tuner works. The hardware video overlay supports far not all graphics cards. A PCI slot is utilized, and at least one hardware interrupt is taken (though they are always need badly). (But not all the weak points listed above are peculiar to tuners integrated into AGP video cards.) Compression and sampling heavily load the CPU. Not all even P4 processors are able to cope with high-quality MPEG compression in the real-time mode. Besides, in many organizations employers have no right to install software themselves. 

But there is a way-out. An external console can receive a high-frequency signal from an antenna, detect it, sample and deliver the digital flow to PC. The first models based on this principle were connected to PC via a parallel port and displayed an uncompressed image of 160x120 and 8 or 16 bits per pixel (at that time it was a real progress :). External TV tuners appeared with a USB bus. Today the market offers solutions with the IEEE-1394 and USB 2.0 interfaces. But in this case quality can be limited by throughput of the interface used. In case of USB 1.0 the maximum throughput is 12 Mbit/s, and it is divided equally between all devices on this bus (mice, keyboards, scanners, printers, cameras etc.). Actually, one device gets 1.5 Mbit/s. Believe me, it's too little for image transfer. A TV tuner compresses images, - for example, Aver TV USB fulfills MJPEG encoding and then transfers data via USB 1.0 at 1.5 Mbit/s to PC. A processor must only decode MJPEG (it's quite simple for modern CPUs) and transfer it to a graphics card. But the actual quality is not satisfactory. A real resolution when frames do not drop out and full color is supported (24 bits per pixel) is 352x288 at most. Tuners of the IEEE-1394 interface (alias FireWare, i.Link) have it better. It supports up to 400 Mbit/s and utilizes DV compression (it is actually MPEG-2 with intraframe compression). But I haven't seen such models yet.

Are there any other ways to watch TV on a PC monitor? As you know, all modern CRT monitors (starting from VGA) are multifrequency; they can automatically tune to any frequency of the frame and line scanning, with any sync polarity and any field interleaving. For example, they can provide 15.625 kHz of the line scanning and 50 Hz of the frame scanning with field interlacing. Such parameters are very similar to SECAM and PAL standards. But after the radio-frequency module a signal is composite. I.e. one cable carries both synchronizations, a black-and-white signal, and two color-difference components R-Y and B-Y. Ideally, a monitor needs 5 cables (for frame and line syncs and 3 color components: R G B). Some expensive monitors can receive a full color TV signal to the frame sync input, and decode it then. But we are looking for a budget solution. It means that a composite signal, before it's applied to a monitor, must be split into all these components. They must be adjusted to TTL levels complying with the standard and applied to the monitor! All this can be done with one popular chip. And you PC screen can be used as a normal TV monitor independent on the computer itself. If you add all features peculiar to modern TV boxes (remote control, screen menu etc.), it will be a very competitive solution. Let's call it a G1 tuner, or first-generation tuner

But we started with a PC console, not with an independent solution. Let's find a computer's place there. Such tuner can be "inserted" in between a PC case, monitor and speakers with VGA inputs/outputs and mini-jack connectors, and signal sources can be switched with relays or integrated circuits. When the device is off, it transfers a PC signal, and when on, it generates independent image and sound. However, you can't watch TV and work on a computer simultaneously, have a TV program in a window, sample a video signal in PC. But anyway, this is a feasible solution.

But LCD panels do not have line interlacing in input signals, and the minimal frame scanning they support is 56 Hz. The G1 tuner won't work with such monitors. But integrated circuitry can help it. We can decode a RF signal, convert it into a digital flow according to the ITU-R BT656 standard, then convert interlaced scanning into progressive one in a special chip, and add a screen menu and other user functions (scanning, PIP, color processing etc.), and generate a classical VGA signal from a digital flow with a graphics chip (similar to one used in graphics adapters); sometimes it can be done with an ordinary DAC. Actually, this is a normal multichip computer with a processor, RAM, ROM, graphics adapter, I/O controllers and several chips for digital video stream processing. At almost all stages a video signal has a digital form. This is what we would call an external G2 tuner.

What is this tuner able of? It's difficult to speak about all devices with such principle of operation, but I'll try to summarize their capabilities anyway. 

Image quality is superb. Such architecture contains minimum analog stages which make the largest impact on quality; an RF module is moved away from the "noisy" power supply unit, and if an input signal is separated into components and applied thus, quality is even better. Also remember that quality of PC monitors (both CRT and LCD) is better than that of a traditional TV tube (for example, in resolution, brightness range, color depth). 

All tuners that I know have a remote control, external inputs and some even have analog TV outputs. Many models are equipped with a clock and a stopwatch, and can work according to a time-table. Moreover, all of them can be used independently of a computer. This is the most valuable feature of an external tuner. What is it for? 

If you have an old VGA monitor remained after PC upgrading, just connect it to a tuner and speakers and you will get a full-function TV set! One more TV set at home is always welcome. 

Do you want a TV in the kitchen but there is not enough space? Just buy the cheapest LCD panel (better those with a wall hook mount and speakers, and which is rather thin), attach a tuner and hang on the wall. Such TV can be a good solution for a home and office (Samsung 152S looks perfectly above my dinner table). Some panels are less than 2-3 cm thick  at the price less than $300. The overall price of such system won't be higher than $400. Just compare it with wall LCD TV sets from Sony or Panasonic which are well over $1000. 

Most LCD panel consume +12V, and all tuners need not more than +6..12 V. That is why you can build a compact portable TV for your car, trips or picnics (again compare with brand-name portable TV sets. 

Video projection systems and plasma panels are often used in offices, demo halls, showcases, and training units. Most of them have both low-frequency video-ins and VGA inputs. If you need a TV signal to get on a screen, you have to connect a TV box or VCR to the projector and use its tuner for reception. In this case an external TV tuner seems to be a cheaper solution which also ensures higher image quality. 

Finally, PC video editing is getting more popular with home users today. The process must be nonstop controlled on the monitor. But its monitoring on a PC screen in the overlay window is very poor, and not all devices support overlay. A good TV monitor is not cheaper than $1000. In this case an external tuner can help again (if it has external LF inputs).

Now, before we study certain models, look through the technique used for estimation of TV tuners.

Unlike most PC devices, TV tuners are placed on the foreground because they have controls, commutation and monitoring elements. That is why appearance is very important for a tuner.

Now let's turn to tuner's main function - TV broadcast reception. Here, it's very important to estimate how well a given tuner is adapted for local broadcasting conditions. At present, the frequency grid is divided into air  FG (a signal goes from a TV transmitter to a TV receiver via radio air) and cable FG (a signal goes only through cable). To prevent crosstalk the frequencies in these grids must be separated. But here it works differently. A main signal comes to a district TV company by fibre-optic cable. The company adds its signal and several decimetric channels, and sends it all that further to a regional TV company. In its turn, this company adds its own signal, signals with radio frequencies, and distributes the resultant signal in the region via a coaxial cable. As a result I got an impressive mixture in my flat which contains metric waves in SECAM in several copies, decimetric channels (sometimes of unknown frequencies), converted satellite channels and so called technological ones. Signals added by the regional TV company come in PAL (though we have SECAM), and some satellite channels have no color or sound (or sound is too high or too low). Some carrier frequencies do not fit into either grid. The saddest thing is that all channels often jump to other frequencies or disappear, or new channels pop in. 

There are also some troubles with sound. By our local standards (in Russia) the audio carrier frequency must be moved up by 6.5 MHz from the video one, while in the rest of the world the frequency spacing makes 5.0, 5.5, and 6.0 MHz. 

That is why tuners should be adapted for local conditions. They must be able to tune to fixed frequencies and scan the whole range. 

A tuner records all channels found into certain cells (which, as a rule, are  directly connected with buttons of a remote control). In my case the tuner used selected cells (2, 4, 5, 6, 7, 9, 12, 17, 36, 54, 82, 85, 91 etc.) and caught some noise between them. Of course one would want to relate "useful" channels with the first buttons (to make it more convenient to switch between them) and delete the rest. That is why I prefer those tuners which support button reassignment. Unfortunately, not all models are able of it.

Since the only monitoring means in an external tuner is a PC screen, the screen menu should have multilingual and detailed information. 

Additional inputs and outputs. They are not vital, but a tuner supports more functions with them. It should have both composite and S-Video inputs. The latter ensures higher quality of images due to separate channels of brightness and color. If a tuner tested has an analog video-out (at least, composite), it can also be connected to monitoring devices different from a PC monitor. It's desirable to have a through audio channel. 

A remote control is the basic (and often the only) control means. It should have buttons arranged conveniently, and be equipped with shortcut buttons for auxilary functions. 

Teletext is also a plus for a tuner.

Now, when we are through with the theory and fields of applications of external PC tuners, we are ready to study definite models. Stay tuned! :)

[ To be continued ]


Mihail Androsov  (nle@ixbt.com)

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