Difference between revisions of "DVB-T Reception Problems"

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DVB-T Reception_problems

Introduction

A number of mythtv users have experienced DVB-T reception problems. These can be particularly tricky to fully diagnose and this wiki is intended to discuss some of the major issues.

This article is unashamedly biased towards DVB-T off-air signals in the UK as it reflects the experiences of the original author but other contributors are invited to improve it and make it more generic.


Typical Symptoms

Picture pixellation, odd sounds, failure to tune in at all and intermittent recordings can all be symptoms. There will be no single 'fix all' solution and an understanding of the transmission network in your area, your locality, your installation and of Mythtv itself will be necessary to resolve the problems.

Transmitter Issues

Which is your local transmitter?

How far away? Is it the only one? What sort of signal could you expect? All questions probably best answered by a postcode to signal predictor site. In the UK, try http://www.ukfree.tv/transmittersmenu.php which has coverage maps and path profiles. The same site will also document engineering work. A glance at neighbours' aerials might also give an insight into the sort of installation needed.

Terrain

Do you have a clear view of the transmitter or are there any local conditions which could degrade the signal? Is there a body of water, hills, buildings or aircraft in the area which could obstruct or cause signal reflections and degradation of the signal? Any of these may need very careful placement of aerials to compensate for these issues.

Aerial

Is the aerial suitable for the signal you hope to capture? Do you need high gain or is your signal very strong? Is a wideband model necessary or can a single band one with higher gain be used? Where is it located? Which polarisation? Generally horizontal for main transmitters, vertical for relays. Is it similar to your neighbours' equipment?

Some aerial suppliers have very informative web sites - a uk favourite is http://www.wrightsaerials.co.uk/ which combines humour with information.

Distribution

How do you get the signal from the aerial to your receiver(s)? Do you have good quality cable with well made connections? Do you have outside cabling subject to UV degradation and water ingress?

Signal quality

Your tuner card(s) may not be as capable as your TV at handling either weak or very strong signals.

Overloading

Some users have experienced a very strong signal which overloads their receiver. This has been reported following a transmitter change which boosted the power, and in another following a software upgrade which improved reception with improved drivers. The solution in both cases was to fit an attenuator or simply a splitter.

Weak signal

This can occur due to deterioration of the installation (eg storm damage or water ingress) or to an inadequate installation in the first place. Signal quality (signal to noise ratio) is more significant than signal strength. Solutions may include a better installation, or inclusion of amplifiers close to the aerial. Some tuner cards have a low noise amplifier (LNA) which can be switched on by a software change, but some of these (eg Hauppauge T500) require the system to be cold booted (ie power down, switch off at power socket for about 20 seconds, then reboot) for the change to be adopted.

Interference

There are many potential sources of interference such as HAM radio, taxi services and transmitting devices in the house such as moble phones, all of which are likely to start and stop. Interference from other TV transmitters, Tetra, existing phone services and the forthcoming G4 services are more likely to be continuous.

Tetra

The emergency services communications system operates at 390 ~ 400MHz. Note that in the UK the lowest TV channel is 21 at 471MHz and superheterodyne receivers are sensitive to interference 72MHz away so filtering may be needed. See http://www.wrightsaerials.co.uk/articles/tetra.shtml

G4

The regulator in the UK (Ofcom) has indicated that with the roll-out of G4 mobile phone services in the 800MHz region a million viewers in the UK may need aerial filters and that 10,000 will need alternatives such as satellite or cable. Channels 52 to 60 are most likely to be affected. For a general release see: http://www.bbc.co.uk/news/technology-17125468

To locate nearby transmitters see: http://www.sitefinder.ofcom.org.uk/search

In 2013 a number of transmitters will have multiplex frequencies changed to make space for G4 and will require retuning of receivers. See: http://www.ukfree.tv/fullstory.php?storyid=1107052059

Careful selection and placement of receiving aerials and possible use of filters may resolve the problems.

Mythtv Software issues

The backend log may provide a clue to a problem and will be one of the first pieces of information requested in a forum posting. Messages logged at the start of a recording, the time of failure or the scheduled end time may be relevant and yield clues via a search engine. The standard location for the log is /var/lib/mythtv/mythbackend.log.

Tuning strength meter

Seems to have been lost from Mythtv. Can anyone enlighten?

Quick Tuning

Some subscribers say that turning on quick tuning has improved Mythtv ability to receive signals. [Why?]

Loss of a whole multiplex

Some have experienced the total loss of a whole multiplex following a change of transmitter line-up during channel re-allocation. The change will typically have been preceded by warnings about the need for a channel re-scan.

The cause might be that new frequencies are used which are incompatible with the aerial setup (eg you now need a wideband rather than a 'group' aerial), the transmitted power having been boosted and needing attenuation (or less amplification) before the receiver, or simply that a fault has occurred at the same time such as water ingress in cabling.

In other cases, it may be due to a software limitation. This problem was noted as far back as June 2009. See http://www.mythtvtalk.com/uk-digital-switch-over-my-experience-11622/#post49900

The same problem could be invoked by a house move from one transmitter footprint to another.

The great puzzle is that your TV will pick up all channels after a rescan and software other than Mythtv on the same hardware will work fine. A fresh installation of Mythtv will also work. A 'mature' installation of Mythtv though will fail to recognise a whole multiplex. This might be a cultural blessing if it merely eliminates shopping channels, but will be a real disaster if it zaps prime channels such as all the main BBC channels.

Why does this happen?

The following is a hypothesis based on interpretation of the symptoms. There are two sorts of information needed to be extracted during a channel scan to allow a receiver to work:

a) Modulation parameters associated with the multiplex such as QAM, bandwidth and guard interval which are necessary to extract the data stream from the signal. Possibly these need to be set in the receiver card/chip as part of the tuning process.

b) Parameters such as channel names and numbers which are extracted from the stream.

During the first ever channel scan the multiplexes and their modulation parameters will be 'discovered' and the Mythtv database populated with them. On subsequent scans, Mythtv will rely on data in the database rather than learning it afresh. It will thus have wrong parameters and fail to unravel the data stream should your broadcaster have changed them. This will result in failure to tune the multiplex and loss of a whole raft of programs. For program to multiplex mapping in the UK see: http://www.ukfree.tv/allchannelsmuxes.php

It is possible that this problem also affects other technologies such as DVB-S, DVB-T2 or cable.

Code fix

The change of parameters at a transmitter is a rare event and moving house is an expensive one. Like solar eclipses, neither of these is easily repeatable and so testing of this condition and a definitive solution ("now it fails; now it works") must be very difficult for a developer. However, this might fix it: http://code.mythtv.org/trac/changeset/a32d5e31ad2833c6e55f3161732faa7fe2f00cfc/mythtv

Backend setup fix

Deleting all channels, all video sources and all capture cards in backend setup, then recreate them before re-scanning will fix the problem. It may help to note all settings first (camera?).

SQL fix

No doubt there is an sql solution to clearing multiplexes and their tuning parameters from the database. If you have the necessary skills do please provide one!

Other issues

This list of issues is by no means exhaustive. Please feel free to add to this wiki and improve or correct it.