The Realtek 2832U

In my spare time (lol!) I have been fooling around with one of those RTL 2832U dongles and a bit of software.  For those that don’t know, the RTL 2832U is a COFDM demodulator chip designed to work with a USB dongle.  When coupled with an R 820T tuner a broadband RF receiver is created  There are many very inexpensive versions of these devices available on Amazon, eBay and other such places. The beauty of these things is that for around $12-30 and a bit of free software, one can have a very versatile 10 KHz to 1.7 GHz receiver.  There are several good software packages for Windoze, Linux and OSX.

The one I recommend for beginners is called SDR-Sharp or SDR#.  It has a very easy learning curve and there is lots of documentation available on line.  There are also several worth while plugins for scanning, trunking, decoding, etc.  At a minimum, the SDR software should have a spectrum analyzer, water fall display and ability to record audio and baseband PCM from the IF stage of the radio.

Some fun things to do; look at the output of my reverse registering smart (electric) meter (or my neighbor’s meter), ACARS data for the various aircraft flying overhead, a few trips through the EZPass toll lanes, some poking around on the VHF hi-band, etc.  I also began to think of Broadcast Engineering applications and a surprising number of things came to mind:

  • Using the scanner to look for open 950 MHz STL frequencies
  • Inexpensive portable FM receiver with RDS output for radio stations
  • Inexpensive Radio Direction Finder with a directional antenna
  • Inexpensive Satellite Aiming tool

Using SDR sharp and a NooElec NESDR Mini+ dongle, I made several scans of the 945-952 STL band in a few of our markets.  Using the scanner and frequency search plugin, the SDR software very quickly identified all of the in use frequencies.  One can also look at the frequency span in the spectrum analyzer, but this takes a lot of processing power.  The scanner plugin makes this easier and can be automated.

950 MHz STL frequencies, Albany, NY
Analog and digital 950 MHz STL frequencies, Albany, NY

I also listened to the analog STLs in FM Wideband mode.  Several stations are injecting their RDS data at the studio.  There is one that appears to be -1500 Hz off frequency.  I’ll let them know.

Next, I have found it beneficial just to keep the dongle and a small antenna in my laptop bag.  Setting up a new RDS subcarrier; with the dongle and SDR# one can quickly and easily check for errors.  Tracking down one of those nasty pirates; a laptop with a directional antenna will make quick work.

Something that I found interesting is the water fall display for the PPM encoded stations:

WPDH using RTL 2832U and SDR Sharp
WPDH using RTL 2832U and SDR Sharp

Not only can you see the water marking on the main channel, you can also see the HD Radio carriers +/- 200 KHz from the carrier frequency.  That is pretty much twice the bandwidth allotment for an FM station.

WDPA using RTL 2831U with SDR Sharp
WDPA using RTL 2831U and  SDR Sharp

Those two stations are simulcasting.  WPDA is not using Nielson PPM nor HD Radio technology.  There is all sorts of interesting information that can be gleaned from one of these units.

Aiming a satellite dish at AMC-8 can be a bit challenging.  That part of the sky is pretty crowed, as it turns out.  Dish pointer is a good general reference (www.dishpointer.com) and the Dish Align app for iOS works well.  But for peaking a dish, the RTL 2832 dongle makes it easy to find the correct satellite and optimize the transponder polarization.  Each satellite has Horizontal and Vertical beacons.  These vary slightly in frequency, thus, but tuning to the correct beacon frequency, you can be assured that you are on the right satellite.  All of the radio network programming on AMC-8 is on vertically polarized transponders, therefore,  the vertical beacons are of interest.  Here are the vertical beacons for satellites in that part of the sky:

Satellite Position C band Vertical beacon (MHz) L band (LNB) Vertcial beacon (MHz) Comment
AMC-8 139W 4199.5 949.25
AMC-7 137W 3700.5 1450.25
GOES15 135.4W 2209.086 N/A NOAA WX
AMC-10 135W 4199.5 949.25
Galaxy 15 133W 4198 949.00
AMC-11 131W 4199.5 949.25
Galaxy 12 129W 3700.5 1450.25

For those in the continental United States, there is not much else past 139W, so AMC-8 will be the western most satellite your dish can see.  Of course, this can be used in other parts of the world as well, with the correct information. Bringing a laptop or Windows tablet to the satellite dish might be easier than trying to drag a XDS satellite receiver out.

AMC8 vertical beacon output from LNB
AMC8 vertical beacon output from LNB

In order to use the RTL-2832U, simply split the output of a powered LNB, install a 20-30 dB pad in between the splitter and the dongle.  Using the vertical beacon on 949.25 MHz, adjust for maximum signal.

Some other uses; look for the nearest and best NOAA Weather radio station.  Several times the local NOAA weather station has been off the air for an extended period of time.  Sometimes, another station can be found in the same forecast area.  Heck, couple these things to a Raspberry Pi or Beaglebone black and a really nifty EAS receiver is created for NOAA and broadcast FM.  One that perhaps, can issue an alarm if the RSL drops below a certain threshold.

I am sure there are plenty of other uses that I am not thinking of right now…

Unbalanced to Balanced Audio

There is a large number of things that amazes me on an almost daily basis.  To wit: a local mom and pop radio station called me because they couldn’t get their computer program to work right.  I decided that I’d give them an hour or two, in exchange for my hourly labor rate, and see if I could fix their problem.  The issue at hand was loud hum and other noise on the input source.  I knew before I even looked at it that the likely culprit was a ground loop.

It was worse than I imagined, with several unbalanced and balanced feeds improperly interconnected, line level audio going to a microphone level input and so forth.  I explained to the guy about putting line level into a mic level input, something akin to plugging a 120 volt appliance into a 240 volt outlet.  Improperly terminated balanced audio nullifies all of the common mode noise rejection characteristics of the circuit.

In any case, there are several ways to go from balanced to unbalanced without too much difficulty.  The first way is to wire the shield and Lo together on the unbalanced connector.  This works well with older, transformer input/output gear, so long as the unbalanced cables are kept relatively short.

simple balanced to unbalanced audio connection
simple balanced to unbalanced audio connection

Most modern professional audio equipment has active balanced input/output interfaces, in which case the above circuit will unbalance the audio and decrease the CMRR (Common Mode Rejection Ratio), increasing the chance of noise, buzz and so on getting into the audio. In this case the CMRR is about 30 dB at 60 Hz.  Also, newer equipment with active balanced input/output, particularly some brands of sound cards will not like to have the Lo side grounded. In a few instances, this can actually damage the equipment.

Of course, one can go out and buy an Henry Match Box or something similar and be done with it.  I have found, however, the active components in such devices can sometimes fail, creating hum, distortion, buzz or no audio at all.  Well designed and manufactured passive components (transformers and resistors) will provide excellent performance with little chance of failure.  There several methods of using transformers to go from balanced to unbalanced or vice versa.

Balanced to unbalanced audio using 1:1 transformer
Balanced to unbalanced audio using 1:1 transformer

Using a 600:600 ohm transformer is the most common.  Unbalanced audio impedance of consumer grade electronics can vary anywhere from 270 to 470 ohms or more.  The 10,000 ohm resistor provides constant loading regardless of what the unbalanced impedance.   In this configuration, CMMR (Common-Mode Rejection Ratio) will be 55 dB at 60 Hz, but gradually decreases to about 30 dB for frequencies above 1 KHz.

Balanced to unbalanced audio using a 4:1 transformer
Balanced to unbalanced audio using a 4:1 transformer

A 600:10,000 ohm transformer will give better performance, as the CMMR will be 120 dB at 60 Hz and 80 dB at 3 KHz, remaining high across the entire audio bandwidth.   The line balancing will be far better into the high impedance load.  This circuit will have about 12dB attenuation, so plan accordingly.

For best results, use high quality transformers like Jensen, UTC, or even WE 111C (although they are huge) can be used.  I have found several places where these transformers can be “scrounged,” DATS cards on the old 7300 series Scientific Atlanta satellite receivers, old modules from PRE consoles, etc.  A simple audio “balun” can be constructed for little cost or effort and sound a whole lot better than doing it the wrong way.

A brief list, there are other types/manufactures that will work also:

Ratio Jensen Hammond UTC
1:1 (600:600) JT11E series 804, 560G A20, A21, A43
4:1 (10K:600) JT10K series 560N A35

Keep all unbalanced cable runs as short as possible.  In stereo circuits, phasing is critically important, so pay attention to how the transformer windings are connected.

Proper termination of long audio wire runs

This is standard telephone company stuff, however, it would seem that several radio engineers have forgotten this.  I was reading on one forum where an AM station was using 1000 feet of 12 gauge romex to send audio from the studio to the transmitter out back.  The owner was complaining that the audio sounded bad.

Longer wire runs need to be terminated with the characteristic impedance of the cable being used, normally 110 ohms or so for typical audio wire.  This is because impedance mismatches can cause return loss just like in an RF circuit.  Exactly what the effect of the mismatched impedance depends on the length and frequencies involved.  On shorter cable runs of less than 100 feet or so, this usually is not an issue.

The result of return loss is part of the audio energy gets reflected back to is origin (a standing wave), where it mixes with newer audio.  This can cause out of phase issues and usually the result is high tinny sounding audio with distortion in the mid range frequencies.  In other words, it ain’t pretty.  This can really become an issue with digital audio because of the higher bandwidth requirements for high sample rates.  It has always struck me as odd that AES/EBU audio uses XLR type connectors.  An XLR connector does not maintain the characteristic 110 ohm impedance of most digital cable and itself can cause pretty significant return loss. But anyway…

There are a number of options for proper termination:

1.  Transformers are often used to match the impedances of circuits. A transformer converts alternating current at one voltage to the same waveform at another voltage. The power input to the transformer and output from the transformer is the same (except for losses). The side with the lower voltage is at low impedance, because this has the lower number of turns, and the side with the higher voltage is at a higher impedance as it has more turns in its coil.  Western Electric 111C audio transformers were often used in equalized TELCO circuits sending audio over long distances on copper pairs.

WE 111 repeat coil, one of the best such transformers ever made
WE 111 repeat coil, one of the best such transformers ever made

2. Resistive network impedance matches such as H or T or L pads are the simplest to implement. They limit the power deliberately, and are used to transfer low-power signals, such as unamplified audio or radio frequency signals. Almost all digital circuits use resistive impedance matching which is usually built into the structure of the switching element.

H pad impedance matching network
H pad impedance matching network

3.  Active balanced converters using opamps with high input impedances (10 Kohm bridging resistance) that first greatly reduce the voltage, then amplify it are often used an audio circuits.  They have the advantage of active gain control and are often used in conjunction with gain reduction and limiting circuits.

Unbalanced to balanced audio converter
Unbalanced to balanced audio converter

The above diagram shows an active unbalanced to balanced audio converter.  The advantages of such a circuit are active gain controls can be added to set levels.  With additional feedback circuit elements, it can also be used for automatic gain control, gain reduction, limiting and so forth.

For most inter and intra studio wiring, professional audio equipment is designed for 0 dBm 600 ohm balanced audio (AKA line level audio).   Audio cable such as Belden 8451 or like multi-pair cables terminated on punch blocks or connectors works well.  Cable impedances and matching are generally not a design consideration.  Long cable runs, longer than 150 feet or so, do need to be terminated in a high quality audio installation.

Wire terminations

Radio studios involve quite a bit of wiring.  Runs between the console and equipment are pretty straight forward, whatever the connector required for the equipment to whatever the connector required for the console.   When it comes to trunk runs between the rack room and the studio, however, some type of terminating block is required.

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66 block or M block insulation displacement wire termination

This particular cabling installation is for low level signaling, contact closures and the like.  It uses a Belden cable with 37 un-twisted wires which do not follow the standard Western Electric color code.  The color code can be found here.  If it where audio or data, the wires would be terminated differently.  That color code can be found here.  For more information on color codes and pinouts, see this post.

Many engineers use the venerable 66 block or M block insulation displacement termination.  These terminal blocks were designed by ATT to terminate 25 pair 22 through 26 gauge solid wire.  The the original design was rated for category 3 (16 MHz or 10 mb/s) communications standards.  Newer designs are category 5 or 5e compliant (350 MHz or 100 mb/s).  Notice the part about solid wire.  Most audio wire is stranded and as such, the metal fingers on a 66 block will cause stranded wire to spread out loosing contact with the terminating finger.  This causes intermittent connections and audio dropouts, which I have experienced often (before I knew better, I used 66 blocks when building studios).  The way to cure audio dropouts on a 66 block is to heat the termination fingers with a soldering iron.  This melts the wire insulation and gets it out of the way.  In the long run, it is better to use more suitable terminations.

Krone LSA-PLUS 110 type wire termination block
Krone LSA-PLUS 110 type wire termination block

The 110 block is updated version of punch block for high speed networks.  it is also designed for 22 through 26 gauge solid wire.  This is the termination used on category 5, 5e, 6 patch panels and RJ-45 jacks.  They are also formed into block type terminations the size of small 66 blocks.  The 110 block is designed for 500 MHz (1 gb/s) or greater bandwidth.  Krone makes a version of a 110 block called LSA-PLUS which is an acronym that stands for: Lötfrei, Schraubfrei, Abisolierfrei, Preiswert, Leicht zu handhaben, Universell anwendbar, Sicher und schnell.  Which translates to: no solder, no use of screws, no insulation removal, cost effective, easy to use, universal application, secure and fast.  Unlike a standard 110 block, the Krone block is designed for solid or stranded wire.  110 blocks are acceptable for use with AES/EBU digital audio at sample rates greater than 268 KHz as well as gigabit networks and analog audio.

In very old installations, I have seen christmas trees.  This is a wire wrap system where wires are wrapped around metal fingers that form the shape of a pine tree, hence the name.  They were very popular in the fifties and sixties and only work with solid wire.  It is also time consuming work and requires special tools and skills.  Wire wrapping is a bit of a lost art.

Christmas Tree wire wrap termination block
Christmas Tree wire wrap termination block

Screw barrier strips have been used to terminate audio cables from time to time.  I wouldn’t consider this method because it is too time consuming, takes up too much space and is difficult to label.

ADC ICON wire termination block
ADC ICON wire termination block

ADC makes a good termination block called ICON (Integrated Cable Organization Network) which uses QCP (Quick Connect Panel) connectors.  the connectors are small square devices that are insulation displacement termination (like 66 and 110 blocks) but require a special tool to “punch down.”   This particular type connector is well suited for stranded wire from 22 through 26 AWG.  QCP connectors are also used on some of ADC’s patch panels and other audio products.  Like any other termination technology, they are only as good as the person punching down the wires.  QCP connections are small high density devices, I have seen them get mangled by a someone in a hurry who got his punch down tool across two of the terminals by accident. ICON blocks can be used for digital audio, however, they do not maintain the 110 ohms impedance of most digital type audio cables (neither do XLR connectors, by the way).  This can lead to some return loss, which on longer cable runs can cause problems.

Radio Systems Studio Hub wiring diagram
Radio Systems Studio Hub wiring diagram

Radio systems prefers RJ-45 connectors with Category 5 cable, something they call Studio Hub.  These are 110 blocks as noted above, but designed primarily for computer networks.  Radio Systems discovered that the impedance of most audio cable is very close to that of computer network cable, audio cable is designed for 110 ohm impedance vs. computer network cable which is designed for 100 ohm impedance.  Therefore, RJ-45 connectors and shielded or unshieled twisted pair work well with balanced professional audio, either analog or digital.

For analog audio wires, ICON blocks seem to be the best, most secure high density termination system.  In all my years of using them, I have never had a connection go bad.  110 block and other category 5 or 5e systems also work well.  For digital audio, Krone blocks or 110 blocks need to be used in order to maintain the full bandwidth characteristics of the cable being used.  Using in appropriate cable and or terminations in digital audio circuits often leads to impedance mismatches and high return losses in the system.