[BC] HD dropouts

[Brian Urban]

OK no one on any of the lists is talking about problems with the HD signal
dropping out.  The grapevine is full of tales of problems.  Rumor has it
that some installs wont even work with T1s between the studio and
transmitter, yet others work fine.  I wrote the following report this
morning which opened a real can of worms both internally and externally.  I
would be interested hearing about any REAL WORLD problems that may be
attributed to HD latency issues.


The HD radio transmitter and antenna system are installed and operational.
HD processing equipment, including the importer/exporter, audio processing,
and second channel insertion gear has been installed at the studio.   We are
currently experiencing momentary transmitter shutdowns due to loss of HD
radio data from the studio.  These transmitter shutdowns are continuous,
occurring every few seconds.  Most muting is for 1 second or less, with a
few incidents lasting up to 5 seconds.  The transmitter will not run for
more than a minute without muting.

HD data is passed from our studio to the transmitter via Moseley Lanlink
radios. Signal strength at the transmitter is well above Moseley's
recommended minimum strength (-90 dB) at -60 dB, with a current signal to
noise ratio of -24dB.

There are latency issues with the HD data which causes the transmitter to
momentarily mute.  Ping tests of the system return an average latency of 300
ms, with a long response time of 1200-1300 ms.  Unofficial specifications
for Ibiquity's system (official specifications are exceedingly difficult to
obtain)  call for a latency of 185 ms for HD data.  Latencies greater than
185 ms are not tolerated by the Ibiquity HD software and are treated as data
link loss.

According to Moselsy, the Lanlink radios are functioning to specification,
especially since we are able to pass IP traffic with out problems.  Spectrum
analyzer examination of the Lanlink path showed some high power transmitters
operating starting at 928.9 MHz which is outside the 904-928 MHz band the
Lanlink radios operate in. No in band interference was noted, however, there
was at least one other spread spectrum system operating in band.  Due to the
nature of spread spectrum radio systems, this is not considered a problem.
Bandpass filters have been installed on the Lanlink in an attempt to remove
near out of band interference.  No change in operation has been noted.
-- 
Brian Urban
KUT Radio
University of Texas at Austin
Office 512-471-1085
Cell 512-940-4757