It seems like I start off every article with “back in the old days…” and the reality is that it makes sense when evaluating new equipment to put it in the proper context. Back in the old days when the broadcast facility was completely analog, you could get away with hanging a pair of headphones across a line to hear what was going on. All you needed was a convenient cable that would allow you to bridge on one end, and a way to plug in headphones on the other. The only protocol you had to be concerned with was analog audio.
So once again, segue to the future — or now. Your facility could be nearly completely digital, or at the very least, it has some digital sources and destinations. Now we have at least two protocols to be concerned with: analog audio and digital audio. It'd be nice if you could just hang headphones across the digital circuits, wouldn't it? Too bad it doesn't work that way.
Fortunately for all of us, several manufacturers make devices that will allow you to effectively hang your headphones across that digital circuit. It's just that you have to put their box between the digital line and your headphones. One such box is the ATI DM500 digital audio monitor.
|Performance at a glance|
Battery powered and portable
AES3, AES3-ID and S/PDIF inputs with loop through
D/A converter with line level balanced out
Variable gain headphone output
Multiple error indicators
LED level indicators
One interesting spec is the unit's ability to output a full +24dBm from its balanced analog output. This was something I was anxious to prove.
The first thing I did with this unit was to compare its level readings with a more expensive, laboratory-grade AES test set (the Audio Precision dual-domain). Each channel has a bar graph on the front of the unit made up of six LEDs. That doesn't provide a lot of resolution, of course, but it gives you an idea of where the level is — it isn't for measurement purposes anyway. Mainly you want to know if you have audio, and that you aren't too close to 0dBFS. I found that the level indications of these LEDs agreed with our other instrument.
I also decided to calibrate the analog outputs for a specific relationship to the peak of headroom for the digital signal; it makes sense that the line level outputs correspond to the audio level encoded in the AES data stream after all. I set the analog output level to be +23dBm when the encoded audio level was set for -1dBFS. Increasing the encoded audio level to near 0dBFS obviously also increases the line-level out; I noted no clipping in the analog outputs. This is impressive for a portable, battery-operated piece of equipment.
After testing the unit on the bench it was imperative to put it to use. It comes with a shoulder strap so you're not forced to set it in a rack somewhere while using it. I slung the unit over my shoulder and headed into the Clear Channel master control (MCR) here in New York to see what I could learn with it.
The easiest source to connect to was the AES output from a Belar FMSA-1. The DM500 indicated this output has a sample rate of 48kHz and had no digital errors; my ears told me this audio was still emphasized. The DM500 has a headphone output that can get very loud — in fact the user manual cautions against keeping the volume up too high for too long. I found that I generally had the gain control throttled down to about 9:00, and that was plenty of level with a pair of Sony MDR7506 headphones.
While the unit was in my possession for this evaluation a particular situation came up in our MCR and the DM500 proved its worth. We were in the process of evaluating an issue with our router affecting the audio transmitted over one of our HD Radio signals.
We wanted to decode the audio and study phase relationships between the channels, but at the same time we didn't want to completely interrupt the air chain. The DM500 facilitates this easily because of its loop-through capability. We set the AES input for bridging, and then patched it into the digital patch bay; we then took the looped output and sent that back to the same patchbay, which inserted the DM500 into the air chain.
The decoded analog outputs were routed over to a scope so we could evaluate the Lissajous pattern.
The loop through capability of the DM500 is especially useful, and it works with the balanced input/output, the BNC input/output, as well as the S/PDIF input/output.
The battery life of the unit seems quite good — the battery pack is made up of four 9V batteries. There is a short cable that runs from the battery pack — which slides on to the bottom of the unit — and plugs in to the DM500. It probably would be a good idea to yank the cable out when you set the unit on the shelf to maximize the battery life (in case you forget to turn the DM500 off when you are done using it). Alternatively you could use a 24Vdc power lump. I noted that the analog outputs, while clean sounding, had a little bit of noise in them. Nothing to be too concerned about, but it did show up in the Lissajous pattern mentioned above, and it's audible in the headphone out.
Overall I would rate the DM500 as very useful around the radio station facility. It seemed obvious to me it was designed by someone who knew what the end users were looking for and expected. It appears to be made very well, and I would expect that it could be inserted and left in an air chain with very little to worry about (except keeping it powered). The input/output connectors are the gold-flash type.
When looking at test equipment for use in and around the radio station facility I think it would be very worth your time to look at the ATI DM500.
Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Email:
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