APT Multi-Channel MPX Codec - Rob Meuser
APT Codec Selected for Pioneering New Digital MPXoIP Studio Transmitter Link for KBLT
Engineaux Inc is dedicated to implementing new technologies and, as CTO, it’s a real passion of mine to get involved in cutting-edge deployments.
In this role, I recently designed and specified equipment for one of the first Digital MPX over IP studio to transmitter links in the United States, for Rice University’s KBLT (K-TRUE) FM station.
KBLT (K-TRUE) were looking to set up a completely new connection; searching for the best way to use existing fibre infrastructure for the STL link while meeting IT standards. We wanted the full MPX baseband to be passed from the studio via IP over fibre. This was, in part, to keep active devices at the Transmitter Site to an absolute minimum.
To my knowledge, there are currently only 3 codec units available offering digital MPX over IP, and we had to choose which one was most suitable for our needs. While cost is always a consideration, we also had to look at the features and overall effectiveness of the solution. In the end, we selected the APT Multichannel Codec. It was competitive on price, offered the reliability we required and critically, it offered SNMP features for remote control. This eliminated the need for a separate remote control system which lowered the cost of the total solution and was also in keeping with our goal of minimizing active equipment at the transmitter site.
We used the composite AES digital multiplex option on the APT Multichannel codec with studio-generated RDS embedded in the digital composite. GPIO is also used.
We did initially consider using SureStream technology to strengthen the reliability of the link but then learned that there was only one fibre path to the transmitter site. Since this is dedicated, point-to-point dark fibre, the single path is highly reliable. However, if there were alternate paths such as two diverse IP radio links, I would recommend SureStream in this application for seamless diversity.
I understand that this deployment was the first APT system in the US to use the digital MPX option, and I believe there are no more than 6 currently operating in the country at present.
Our experience installing the APT system was very straightforward, and when we did have an issue with option licensing, Tony Peterle in the Miami office went above and beyond to straighten it out.
Whilst developing their MPX option I learned that APT worked directly with the German ARD Group in developing the active OMC (Over Modulation Cancellation) Technology.
OMC means that if the MPX IP stream is interrupted due to a “Loss of Connection” event or dropped packets, then there is no possibility of an over-modulation of the RF carrier due to sharp signal transients caused by the network.
The OMC algorithm prevents the occurrence of high energy peaks by means of a moving average filter. It suppresses any harmonic frequency above the 15kHz spectrum, and the RF deviation will not exceed the allowed limits (2%). Crucially the OMC algorithm only acts on dropped packet(s) and does not alter the original signal in any way in normal operation.
So far, the APT units have been flawless, stable and reliable and, in the long term, the use of Digital MPX is helping Rice University make significant savings by reducing the manpower required to maintain the plant.