Behringer UCA202 External USB Sound System For PCs.

The UCA202 is an inexpensive but high-performance USB-connected external "sound card" for PCs.  Originally intended for pro and semi-pro audio applications with mixers, PA amplifiers, etc, it is also ideal for amateur radio "sound card" digital mode operation.  It consists of a cigarette-pack-sized silver plastic box attached to a 3-foot (1 meter) USB cable.   

Unlike most computer sound systems that combine left and right channel audio on 3.5mm 3-conductor "mini-jacks", the audio inputs and outputs of this device are on white/red pairs of individual RCA jacks. The separated channels make using this device with SDR I/Q-input applications, or with dual-port "soft TNC" software like the AGW Packet Engine or the UZ7HO Soundmodem much more convenient. You don't need to resort to kludges of "Y"-cable splitters to separate the two channels.

The technical specs of this device are astounding for an inexpensive audio device. (Around USD $30 at Amazon.com - search for "Behringer UCA202")  The frequency response is essentially flat from 5 Hz to 20,000 Hz +/- .5 dB.  Adobe Audition shows it's noise floor to be 95 dB (!) below peak output on it's wide-range "VU meters".  The device is capable of 96K 16-bit sampling rates. 

The EasyPal "digital SSTV" sound card program is a useful tool for testing the performance of computer sound systems.  This program use 16 QAM audio subcarriers in parallel within the 300-2400 Hz voice passband.  EasyPal is extremely sensitive to noise and intermodulation distortion in the audio systems and radio link.  The program provides a display of intermodulation distortion in dB. (Given that the application uses digital transmission techiques, I'm not sure how it arrives at an analog value like IMD - perhaps it converts bit-error rate to an equivalent IMD value reflecting the quality of the audio channels of the radios involved.) 

Normally, the typical IMD seen over-the-air on HF/SSB links is around -10-to-13 dB and around -16-to-20 dB on carefully adjusted VHF/FM links with good sound cards.  Connecting two good soundcards together directly with a 3.5mm patch cord (no radio modulators/demodulators/link noise in the path) will yield 25-28 dB IMD or so.  Two Behringer '202's connected directly together yielded a staggering -45 dB IMD.  Over the air with a pair of Kenwood TMG-707 transceivers on 2 meters FM, I was still able to exceed -35 dB IMD with careful tweaking of the transmit modulation level. 

The UCA202 has "line-level" inputs only; there are no low-level mic inputs.  If you wish to use unamplified mics (for applications like EchoLink or Skype), you will need to provide an external mic-to-line-level preamp.  In addition to the fixed line-level outputs, a 3.5mm stereo mini-jack provides a "monitor" output, adjustable with a physical thumb wheel control for headphones. A tiny slide switch (not visible in the photo) allows the analog input to be passed directly to the output for monitoring and testing the input, without having to route audio into and then out of the computer.  The headphone output could be useful if a hardware-adjustable audio output (i.e. "transmit") is required in ham sound card applications.

A TOSlink optical output is provided for totally  ground-loop-free  hum-free  connections to modern stereo amplifiers and AV receivers.    Anyone that has been frustrated with the hum, hiss, hash and buzz from ground loops, computer clocks and DC-DC power converters on motherboards that one encounters when connecting PCs to good sound systems with metallic patch cords will appreciate this.  (Normal cables unavoidably connect the ground systems of the computer and audio gear together creating ground loops; i.e. what the British and Europeans refer to as lack of "galvanic isolation".)  This feature is wonderful for playing streamed Internet radio stations and other media audio over good external sound systems.

Plugging this device into Windows 2000, Windows XP or Windows 7 causes Windows to "auto-magically" enable the unit as the generic "USB  Sound System" or "USB Audio Codec".  Any application that can select specific sound cards can then use it. 

For ham interface applications, the sound card interface needs to provide an adjustable-level receive output.  Or a simple fixed voltage divider of two resistors could adjust the level.  Due to this device's incredible dynamic range and low internal noise floor, the input level is not at all critical - pretty much anything under about 2 volts peak-to-peak can be fed into it as-is with no problems.  [Note that you DON'T want the interface to pad the receive level down to mic level for the typical computer MIC input. You would be throwing away 20-30 dB of the potential dynamic range of the UCA202.]

Behringer also offers a nearly-identical looking device, the UFO202 at the same price. The UFO202 lacks the TOSlink optical output, but adds an RIAA-equalized magnetic phono input. The slide switch that enabled the monitor input is used instead to switch the input RCA jacks between flat-response line-level input and low-level equalized phono input.  The input-monitor pass-through is permanently enabled on the UFO202.  This allows you to listen to turntable input on headphones or amplified computer speakers attached to the output jacks, without having software running on the computer.

The always-on pass-through monitor audio of the UFO202 makes the unit ill-suited for ham applications.   Receiver audio is looped directly back to the transmitter where it will continually trip VOX devices either in the sound-card interface or inside the transceiver itself.  (It took me an entire evening of aggravated head-scratching to discover why my homebrew tone-keyed interface, described here, went into continuous keyup-unkey-keyup-unkey when I substituted the UFO202 for the nearly-identical UCA202!)

On the other hand, the UFO202 is an excellent low-noise phono preamp.   Connecting a quality classic turntable with a decent magnetic cartridge to the phono input on this device is the definitive way to digitize vinyl records.  The noise floor of the UFO202 is at two or three orders of magnitude lower than the best vinyl records. 

Connecting the low-level output of the turntable directly to this device (instead of using a separate  phono preamp, or the preamp section of a vintage stereo receiver) avoids another AC power supply and the associated headaches with ground loops and AC hum in the captured audio. 

The extremely wide dynamic range of the input allows the digital recording to preserve the high amplitude of vinyl record clicks and pops relative to the desired audio from the analog input, so that click & pop eliminator routines in software like Audacity or Audition can more positively identify and remove them. The results are VASTLY superior to the cheap USB turntables using low-grade limited-dynamic-range audio-to-USB codecs currently on the market.