How Spokane Public Radio gets from here to hear. Part 3

Last time we left off, we had our audio stream ready for signal processing. Processing involves not only regulating the volume and tone quality of the signal; it also prepares the audio for FM broadcast.

A quick explanation of FM is in order. FM stands for Frequency Modulation. The program audio actually instantaneously changes the frequency of the carrier that the transmitter is putting out. KPBX only transmits at exactly 91.1 MHz when there’s no audio. The louder the signal is, the farther we move away from 91.1 MHz. The FCC has strict guidelines as to how far we can deviate from the center frequency. With complex audio, especially music with loud high frequencies, we need to be very careful about the harmonics produced. High energy, high frequency sound can very easily drive our modulation past the legal limits.

Much of the enjoyment of music is due to the difference between the soft parts and the loud parts. This is referred to as dynamic range. At Spokane Public Radio, we strive to provide a good balance between not-too-quiet and not-loud-all-the-time. If we didn’t compress the dynamic range somewhat, it would be nearly impossible to hear softer passages while in your car. This also means we need to put the brakes on the really loud passages in order to stay legal. But that’s only part of the job of processing; it does much more.

Until very recently, the FCC had mandated that enhancements to broadcast technologies had to be backward compatible with existing equipment. The effect of this is that a person listening to a stereo program on a monaural radio must be able to hear the entire program. This is all done by the audio processor.

The processor receives the Left Channel and Right Channel audio signals from the mixing board. After adjusting the volume levels and the tone color, etc, the processor combines the two signals together to produce both a Left+Right signal and a Left–Right signal. The L+R part contains all the audio information and thereby becomes the monaural signal which occupies the center of the spectrum from –15 KHz to +15 kHz around the center frequency. That’s the only part of the signal that a monaural radio needs to receive.

When the station is in stereo mode, the processor produces a very precise 19,000 Hz sine wave (+/– 1 Hz !) This stereo pilot tells your radio that it’s receiving a stereo program and turns on the “Stereo” light. Your radio has a filter to keep this pilot tone out of the audio you hear (15 kHz “brick wall”.) The processor then uses this pilot to produce a second signal at twice that frequency (38 kHz) which is combined with the L-R information. This resultant carrier is parked out at +/–38 kHz from the center frequency.

Out beyond the 38 kHz L–R information, there are SCA (Subsidiary Communications Authority) channels. The Radio Data Service (RDS) at 57 kHz provides station information to radios that are equipped to use it. KPBX uses a 67 kHz SCA to transmit the Evergreen Radio Reading Service for the Blind. It takes a special radio to be able to listen to this service. There’s even a 92 kHz SCA that we can turn on as a backup return telemetry path for the remote control equipment at the transmitter site.

In a previous installment, I promised a discussion of digital audio, so let’s go back to some sound fundamentals When we hear sound, it is because all the objects producing sound are vibrating and producing tiny variations in the air pressure right around them. These pressure variations travel away from the source and combine with all the other sound producers. All of their pressure waves mix, adding here and subtracting there, which results in a single (but very complex) pressure wave that arrives at your eardrum (or a microphone) and causes it to vibrate in accordance.

We talked earlier about protecting this signal from outside interference with shielding and balanced wire techniques. An alternate way of reducing noise is to convert the sound information into code. After the voltage signal is received, it goes through an analog-to-digital (AD) conversion. This consists of taking thousands of measurements each second and assigning a number to the instantaneous voltage of each measurement. The beauty of this is that these numbers are very interference-resistant. They’re all encoded in binary form with each bit being either a “one” or a “zero”. These numbers can then be transported between computers and “reassembled” at the final destination using a digital-to-analog (DA) conversion. As you might surmise, the more measurements (samples) you take per second and the finer degree of voltage variation you can detect, the closer you can get back to the original sound, which is analog. As an example, the standard digital coding used for CDs is 44,100 samples per second. Each sample can have a value of between 0 and 65536 (16 bits).

Okay. So what’s the difference between AM, FM, and HD® radio?
Watch for Part 4.

Twisp Update

As it turns out, the audio problem at Twisp appears to be a satellite system problem rather than a receive problem. We now have information that shows that Omak (and consequently, Oroville) have had the same drops in the audio.
The Toronto office of the satellite company confirmed that they are having intermittent failures with all the programs from Spokane but they don’t yet know what is the problem.
So, there’s little we can do to fix the satellite issue.
The immediate plan is to get working on an alternate delivery system. We will be testing to see if we can get an adequate over-the-air signal from Brewster to feed both Twisp and Omak. The Brewster transmitter was not in place when the Twisp and Omak units were commissioned.
We’ll have to buy some more gear to make this happen but, over the long run, the system will be more reliable if we have two ways of getting the KPBX signal to those transmitters.
In the short term, let’s hope the satellite issue gets resolved sooner rather than later.

Twisp audio “holes”

We’re again having problems at the KTWP transmitter site that serves Twisp and the surrounding area. Late last week, we started getting reports of audio dropouts lasting anywhere from a few seconds to 30 minutes.
All of the transmitter parameters are normal, so that leads us to believe that there’s an issue with the the reception of the satellite signal that brings the audio program to the site.
It can be one (or more) of several things causing the problem. In order of likelihood, they are: satellite dish off prime focus, bad LNB, damaged cable, flaky satellite receiver, something external that is interfering with the satellite signal locally.
I’m sending some test gear over to the area today. When it gets there, it may help diagnose the cause of the problem.
I’m sorry for the interruptions; I know they’re annoying, to say the least. We will get this fixed as soon as we can.

The Fix is In!

We were able to install the new notch filter at the KIBX transmitter site this morning. It looks like that has cleared up the drop-out problem as well as restoring the HD-2 and HD-3 channels.
Little successes like this plus a great view from Black Mountain this morning really add to the enjoyment of this job.

View toward the South

View of Bonners Ferry

KIBX Dropouts

Once again, we are having problems with the 92.1 KIBX transmitter in Bonners Ferry. Local interference at the site is causing the receiver to intermittantly loose contact with the Mothership, KPBX. In theory (and until just recently), KIBX receives all three HD channels from KPBX in Spokane and retransmits the three channels on 92.1.
I traveled to the site last Friday with the site technician in a vain attempt to resolve the problem. We experimented with different receive antennas and amplifiers and thought we had improved the situation, at least temporarily, while awaiting delivery of a notch filter.
Only after packing up and leaving the sight (with some nasty weather blowing in) did we hear the dropouts return. It was too late and dangerous to return on Friday.
So, a notch filter has been ordered and should arrive by the end of this week.
I still don’t know why an existing translator at the site has suddenly started causing us such grief but we will keep attempting to resolve it.
Sorry about the audio interruptions; I know, from personal experience on my drive home, how frustrating that is.

How Spokane Public Radio gets from here to hear. Part 2

When we left off last time, we had a tiny voltage from a microphone to deal with. How do we make use of that? We need to boost that voltage up to a more usable level.

Our modern environment has become extremely noisy, especially electrically. There are radio signals everywhere and power line buzz is rampant. Don’t even get me started on cell phones. (bippitty- bippitty- bippitty -bippitty-biiiiiiiiiiiiiiiiiiiiiiii)

So, how do we boost the tiny signal we desire without also boosting the noise we don’t want? There are two main methods. One is shielding. We surround the wires carrying the signal with a grounded metal shield that drains away the outside signals. In the microphone itself, the metal microphone body provides the shielding. For the connecting cable, we rely on a flexible metal covering that’s built into the cable. This covering is made either of foil or a braided mesh of very fine wires. Shielding is effective at reducing the amount of unwanted signal getting to our microphone wires but it’s not enough. The really heavy lifting is done by using balanced audio. What balanced audio means is that two identical but mirror-imaged signals are produced in the microphone. The cable carries both these signals on two separate wires twisted together inside the shield. The amplifier used to boost this signal is designed so that it will only amplify the difference between these two signals. Since the desired signal is equal but opposite on each wire, the difference between them is twice the original signal. That part gets amplified. Any electrical noise that gets into the cable on its way to the amplifier will be imposed on both the signal wires equally. Since it’s the same signal on both wires, this added noise will be cancelled out by the amplifier.

Balanced audio is used throughout the entire audio chain all the way to the transmitter. It’s even used for digital audio lines to reduce the number of spurious 1’s and 0’s that get in and mess things up. (More on digital audio in the next installment.) Balanced audio is probably the single most important feature that distinguishes professional grade equipment from consumer grade. Professional equipment also has signal levels roughly 5 times higher than consumer equipment.

That being said, our microphone amplifier has only gotten our peak microphone voltage up to about ¾ of a Volt…roughly half that of a flashlight cell. Small as that may be, it’s the reference standard for all the audio gear in the studio. All the CD players, audio playback computers, tape decks and record players are based on this voltage standard. By the way, a phonograph cartridge is really just a specialized microphone. (Yes, we still play records…a lot of records.) The stylus (needle) has a tiny magnet on its tail end. The stylus follows the groove in the record which makes the magnet vibrate in accordance. This magnet is located in very close proximity to two coils (one for left channel and one for right channel). This creates the two tiny voltages that must be amplified and frequency-equalized by the phono preamplifier to bring it up to the standard ¾ Volt level used throughout the station. Since there is no opportunity to use balanced audio between the phono cartridge and the preamplifier, the preamp is located as close as possible to the cartridge, usually within a foot or two. Even so, that’s the one area in the station that’s most prone to noise pickup.

All these audio signals are brought to the mixing console where the Board Operator (on-air host, DJ, etc.) selects which one to pass on to the transmitter. There is a mixing console for each station. The operator can pre-listen to any of the audio sources through separate audio paths that (hopefully) don’t go on the air. It’s up to the operator to make sure that the correct audio gets to the transmitter at the correct volume level.

Reality being what it is, there are some times when one or more of the stations is running in automatic mode. In this case, computers route the correct programs and announcements to the transmitter…but only if we’ve correctly told the computer what to do.

Now we’ve got an audio signal ready for the next step…Audio Processing.

How Spokane Public Radio gets from here to hear. Part 1

This is the first in a series of posts in which I will describe the various systems used to get our great programs to you. In other words, how does our radio station work? I will attempt not to get too geeky but I want to include enough detail to make it interesting.

The general categories will be:
• General overview (this posting)
• Sound sources and routing
• Audio Processing (digital vs. analog)
• AM, FM, and HD® Radio
• Signal transport to the transmitter
• Transmitter and antenna
• Your radio (where the real magic happens)

There will most likely be some jumping around in order to better explain one aspect or another of the systems.

I think the way to start is to talk just briefly about electromagnetic energy. This energy is all around us all the time. Visible light is a form of electromagnetic energy but its frequency is much, much higher than anything we can approach with radio. For example, AM broadcast radio operates from 540 to 1610 kilohertz. One great advantage of operating in this frequency band is that the radio signal can actually bounce off the ionosphere and thereby be received at locations beyond the horizon. This reflection falls apart when the sun is shining on your portion of the ionosphere because the sun’s energy (also electromagnetic) scrambles the ionosphere enough to make it a poor reflector. That’s why you can often receive distant AM stations on those cold winter nights but not during the day. The downside of this is that most AM broadcasters must drastically reduce their power from sundown to sunrise (ever-moving targets) to reduce interference with distant stations operating on the same frequency.

FM broadcasters operate in the frequency band from 88 to 108 megahertz (1 megahertz = 1000 kilohertz). The frequencies from 88 to 92 MHz are reserved for non-commercial educational stations, such as Spokane Public Radio. These frequencies are sufficiently high that the ionosphere is transparent to them and offers no reflection. For this reason, FM broadcasting is line-of-sight. If you’re over the horizon or behind a mountain, the signal won’t get to you. There are some situations where the signal might be reflected back from a higher mountain behind you, but usually that does not provide a usable signal and it often creates interference. (See the banner at the top of this page.) By the way, visible light, when expressed as frequency, is between about 400 to 790 terahertz (mega-megahertz). Light doesn’t go through mountains very well either.

But here’s the real beauty of electromagnetic energy; a magnetic field will produce an electrical field and vice versa. If a moving magnetic field intersects with a piece of wire in a circuit, it will produce an electric current in that wire. Conversely, if a wire is carrying an electric current, it will produce a magnetic field around it.

This is the basis of a standard (dynamic) microphone. Inside the microphone is a very lightweight coil of wire that’s physically attached to a very flexible diaphragm. That coil is located very close to a magnet. When a sound is produced, it causes the air pressure to change at the frequency of that sound. That air pressure change makes the diaphragm (and the coil of wire) inside the microphone vibrate at that same frequency. Since the coil is in a magnetic field, it produces a varying voltage (albeit very tiny) at the frequency of the sound. The louder the sound, the higher the voltage signal produced. Even so, this voltage is only about 1/1000th the voltage of a standard flashlight cell. That’s too small to be very useful at this point.

In the next installment, we’ll talk about how we use that tiny voltage.