Sattelite Uplink Distorting

[shaynesinclair]

Hi Guys,

I'm by no means an Engineer, i'm actually the Music Director for a rather large network of stations in Australia. I have a big interest in Engineering, but forgive me if I don't make much scence!

Our network has two main Hubs that deleiver networked programming. The station I am based at takes the night show from one Hub, then flicks over at midnight to the other Hub for Mid - Dawn programming.

For the last 3 years I have heard some slight distortion (constant) on one of those uplinks. It's only in the left channel. The last station I worked for in the group had their Optimod 8200 smashing out the absolute loudest sound on the dial that brought this distorion out big time. I brought it up with our local engineer, who admitted he could hear it and then then passed the message onto the Cheif Engineer at the Hub where this audio was coming from - he did some tests and came back with the response "Everything looks fine to me"

I know that the distortion is coming from the Hub because you can hear it on every station in the group when they are taking programming from this particular Hub. Music, Speach, Commercails - it's everywhere. Across the other side of the country at the other Hub we receive programming from has a crystal clear sound with no distortion in site - Another reason I beleive it is coming from the OTHER hub (confused?) I have more theries...But I will save them!

Can anybody suggest a reason why this is happening? Where could it be coming from? How it can be fixed?

I just looked in the control room and we are using Starguide decoders (I assume they would be the same encoders too at the Hubs?)

Thanks in advance

Shayne

 

[littlejohn]

Starguides as digital? Then the up/down likely isn't doing it. And the vote is very many to one that the problem is coming from the hub, or the uplink. Swap everything at the hub one at a time with the other system, and see what changes it. Often, heavy processing will point up problems which seem minor at the source.

 

[kenglish]

It doesn't take much to cause "clipping" distortion on a Digital audio link.
Many stations use processors that "normalize" the audio to have their peaks right at 0 dBFS (zero dB, referenced to full-scale, or the "limit" that digital can handle).

We switched our TV audio processor (really, an FM radio processor) to digital mode, and found that out the hard way. The easiest solution was to use a spare "do-it-all" type of box, a Bel Audio AES/EBU Digital Shuffler/Mixer/Router Switch, set to act as an AES/EBU attenuator. We dropped the "full-bore" levels by about 15 dB, which brought it in line with other sources, but you could set it for less attenuation.