HOW THEY DO IT AT PRM

From the Concert Hall to Your Hall

A conversation with Jason Black, PRM's Chief Engineer
December 6, 1995

Note: While much has changed at MPBR (aka PRM) over
the last 20 years, the comments under
II: Processing the Signal are still valid.
January 10, 2016.

I. WHERE THE SOUND BEGINS -

CONTINUO: What are the most difficult events to record?

BLACK: A few years ago we sometimes would record a major event, and would put up several microphones and carry along a 24 channel mixer. But since we got our new stereo microphones it has been a lot easier.

CONTINUO: How effective are the new stereo microphones (these are the ones that use a single stand)?

BLACK: The single stand mike is an AKG Stereo Microphone. It makes good recording so far as stereo goes. In Mara Hall we did have two separate microphones hanging, but have now combined them so they hang from the same cable. They are positioned ("aimed") to produce a stereo result.

CONTINUO: Do you still use LPs? Cassettes?

BLACK: The only LPs we still use are for our Bluegrass shows. We can't, however, play them on the air, as we don't have any LP turntables in the control room. We first put them on DAT tape (Digital Audio Tape - the tape equivalent of CDs).

CONTINUO: How do you receive signals from NPR (such as Morning Edition) and PRI (such as Prairie Home Companion)?

BLACK: That all arrives by satellite. Listeners, by the way, may expect an improvement in sound quality. About two weeks ago NPR converted its satellite from analog to digital, and we also made the same conversion. This will improve signal quality in a way similar to the improvement from the conversion from LPs to CDs. NPR leases space on the satellite (Galaxy 4) to PRI.

II. PROCESSING THE SIGNAL -

CONTINUO: Do you do any volume compression?

BLACK: Yes. That is a must. Classical music. The signal is very wide - because of the wide dynamic range - and yet, because of FCC limitations, we have to trim it to make it fit within a fixed width. We actually do run some limiting and compression on our audio programs; the amounts are about 8 dB of compression and 8 dB of limiting. Compared to other [i.e. commercial] radio stations, however, we hardly use any compression. They use from 25 to 35 dB, and if you watch the needle on the VU meter on your amplifier, you will see that if you have tuned in a rock station the needle basically vibrates around 100% all the time. Your eardrums are also doing basically the same thing. This leads to listener fatigue. PRM's signal, on the other hand, causes wide jumps in the needle, indicating a range from soft to loud.

CONTINUO: What frequency range is broadcast? Are there upper or lower cutoffs?

BLACK: We have a sharp upper cutoff at 15 kHz, because we a portion of the signal must be used to produce the stereo signal.

III. BROADCASTING THE SIGNAL -

CONTINUO: A list of your stations includes at least one that is referred to as a "translator". What is a translator?

BLACK: We have a translator in Hattiesburg. It is actually a receiver and a transmitter. It receives the signal by antenna - from our Biloxi station (90.3 MHz), lowers it (to 89.3 MHz) and rebroadcasts it at that frequency.

CONTINUO: Several years ago at each station break PRM listed all the call letters and locations for all the transmitters. Now we hear only those from the actual station we are hearing, even though there is no studio there. How is this done?

BLACK: Mark Mango and I built this feature in 1989. It is an 8-bit system that uses PROMs to record the voices [a PROM is a Programmable Read-Only Memory computer chip which, because it has no moving parts, is relatively trouble-free].

CONTINUO: How do you manage technical support at the outlying transmitters?

BLACK: Each transmitter has a site supervisor and an assistant who maintain the site as well as any microwave equipment at the site.

CONTINUO: Where is the Jackson area transmitting antenna?

BLACK: We lease space on the Alpha-One (WAPT-TV), Inc., tower. WTYX and WDBD also have space there. It is located near Metro Center.

CONTINUO: How far outside Mississippi can PRM (as a network) be heard?

BLACK: We actually show up in Arbitron ratings for Alabama and Louisiana.

CONTINUO: On Fridays (request day) people often call in from Memphis and even Jackson, TN. How far from each individual transmitter can the signal be heard?

BLACK: The FCC has a standard known as the "1 millivolt contour", roughly a circle around each station within which you should be able to get a full quieting signal. That contour has a radius of approximately 40 to 45 miles around each transmitter. The line is predicted by computer, but the basis is an actual reading on a field strength meter. Beyond that contour competing stations may use the same or adjacent frequencies (by FCC rules). Contours using the same or adjacent frequencies may not overlap.

CONTINUO: Does PRM have any overlaps from competing stations in Mississippi?

BLACK: Not to my knowledge.

CONTINUO: What influences the distance? Weather? Time of day?

BLACK: Cloud cover can sometimes cause skip. Other atmospheric conditions can also interfere.

CONTINUO: What explains "dead" spots, for example on Lakeland Drive just east of St. Dominic's Hospital, and on Monument St. near West Capitol (both in Jackson), where the signal seems to become much weaker?

BLACK: Multipath interference. When you are between a transmitter and a large building so that you get an additional signal as a reflection off the building, it interferes with the primary (non-reflected) signal because it arrives at your antenna very slightly later. We also run two subcarriers on our signal (this would be like two small separate tubes within the main pipe). One of these is for the Radio Reading Service, and these can sometimes contribute to multipath interference.

We are planning to upgrade the Jackson antenna, which is now a vertically-oriented antenna, to one oriented both vertically and circularly. This will improve the signal strength.

IV. RECEIVING THE SIGNAL -

CONTINUO: What dB range is achievable with good equipment?

BLACK: Without volume compression, about 74 dB signal to noise ratio, which is about the best that FM technology can do.

CONTINUO: Is volume compression available as a feature in receivers?

BLACK: Yes. It is especially useful with CD players - with their very wide range - in automobiles, which are generally noisy. It turns on with a press of a button.

Glenn A. Gentry, Editor