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Important.png Note: The correct title of this article is tzap. It appears incorrectly here due to technical restrictions.

tzap is a utility for testing the tuning and operation of DVB-T adapters. Most of this information also applies to azap, czap and szap, the ATSC, DVB-C (cable) and DVB-S (satellite) versions of this utility.

Getting tzap

tzap is part of the dvbutils package. Follow the link for details of how to get dvbutils.


tzap requires you to have first placed a channels.conf file from dvbscan or w_scan into your ~/.tzap directory.

If you have a single DVB device, usage is as simple as

tzap "BBC ONE"

Entires in this file consist of fields separated by colons (:). To find the channel names, look at the first field in the channels.conf file. Typical entries look like this:


The actual lines are longer, but the important part is the first field, the text up to the first colon. This is the name of the channel. In this example, there are two channels, SBS DIGITAL 1 and SBS DIGITAL 2. You must type the complete name of the channel, though you don't need to CAPITALIZE.

The output looks like this

using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
reading channels from file '/root/.tzap/channels.conf'
tuning to 754166670 Hz
video pid 0x0258, audio pid 0x0259
status 00 | signal ffff | snr c000 | ber 00003fff | unc 00000000 |
status 01 | signal 3737 | snr c000 | ber 00003fff | unc 00000000 |
status 1f | signal 3636 | snr ffff | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal 3636 | snr ffff | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal 3636 | snr ffff | ber 00000000 | unc 00000000 | FE_HAS_LOCK
status 1f | signal 3636 | snr ffff | ber 00000000 | unc 00000000 | FE_HAS_LOCK

The "FE_HAS_LOCK" (frontend has lock) messages being the important indicator that your card has locked onto a signal. The other values are important diagnostic indicators.

The short explanation: frontends seem to vary in the way they present these numbers, it seems, so don't break your back trying to get the numbers "perfect". If the snr value is steady and reasonably high, and you can get a consistent lock, your card will probably work.

Testing with cat

You can use the tzap utility to record video. It's very primitive, but it works.

First, in one tty, set tzap going on the channel of your choice

tzap -r "BBC ONE"

The -r is the bit that makes it work. Now leave it running, go to another tty, and

cat /dev/dvb/adapter0/dvr0 > testvideo.mpg

The file should rapidly fill up with data. If it stays empty, something's not right.

Even better, you should be able to play this file through mplayer as simply as

mplayer testvideo.mpg

Interpreting the numbers


This value is pretty self explanatory; it's not actually all that important though. It's also rather misleading as different frontends report different values. My Nova-T reports ffff (100%), but it's being fed by the same signal as my DEC2000-t, which reports 6f45 (43%). Even more confusing, the DEC2000-t reports 84% signal when it's in STB mode.


I've found that the signal/noise ratio is the most important indicator of your card working successfully. It doesn't need to be as high as possible (but higher values are better), but it seems it does need to be steady. My DEC consistently reports a SNR of 8004 (but the STB GUI reports signal quality of 100%). I was trying a poor antenna and I could get a signal on my Nova-T, even with a very poor signal strength, as long as the snr value was steady. Fluctuations in this value are bad news.


The bit error rate presumably displays the error rate for the card. Again, while my Nova-t was reporting an error rate of zero, my DEC2000-t was reporting bffff8b8, despite the GUI on the box reporting 100% signal quality. Again, I think it's more important for this value to be steady than it is for it to be low.


This value reports the number of uncorrected blocks. Low is good. Again, my DEC is producing misleading values here, so steady is as good as low, I think.

Other tools

VLC is useful for visual checks of signal quality.

[wscan] is useful for scanning for channels and producing a channels.conf file