[mythtv-users] Signal strength, Freeview uk.

Nick Craig-Wood nick at craig-wood.com
Mon Jul 2 12:06:44 UTC 2007


Matthew McClement <mythtv at macker.co.uk> wrote:
>  I believe the MythTV wiki covers this, but when it comes to signal 
>  quality having a tune lock with steady snr and ber values is what you're 
>  really looking for. The signal strength at best tells you that the tuner 
>  is getting something vaguely signal-like from your aerial.

Yes, I think that most of the boards put out some AGC voltage in the
"signal" reading which is OK for relative values but not calibrated.

The best way to know how well tuned your system is, is the "ber" value
shown by femon, eg

  $ femon
  using '/dev/dvb/adapter0/frontend0'
  FE: Philips TDA10045H DVB-T (TERRESTRIAL)
  status 1f | signal d0d0 | snr fefe | ber 000008f8 | unc 00000000 | FE_HAS_LOCK
  status 1f | signal d0d0 | snr fefe | ber 00000a7e | unc 00000000 | FE_HAS_LOCK
  status 1f | signal d0d0 | snr fefe | ber 00000962 | unc 00000000 | FE_HAS_LOCK
  status 1f | signal cfcf | snr fefe | ber 00000940 | unc 00000000 | FE_HAS_LOCK

The "ber" value is the number of *corrected* errors per second (in
hex).  This is the raw error performance of the channel and can be
used to calculate the true signal level (Eb/No).

To know whether your system is tuned well enough, make sure you
*never* get any "unc" readings.  These are uncorrectable errors and
will make a glitch in your recording.

The error corrector can deal with a certain level of errors just fine,
however once the error rate gets above a certain rate then
uncorrectable errors will come through.  (That is a simplification -
it is all about probabilities!)

The above shows my worst channel ("ber" averaging about 0xA00) which
very occasionally (say once every 5 hours recording) makes a
noticeable glitch in the recording.  I'd take that value as a maximum
acceptable value for "ber".

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick


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