The Digirator DR2 is the latest Minstrument from NTI, the audio test and measurement company headquartered in Liechtenstein. The Minstrument product line consists of a series of separate test generators and signal analyzers spanning a wide range of digital, analog, and acoustical tests.
These are lab-grade instruments, but are all compact, battery-powered (three AA cells), and rugged enough for field troubleshooting or system testing. The DR2 provides a comprehensive range of audio test signals as well as specialized test sequences for verification and adjustment of Dolby Digital, Dolby E, ProLogic, and DTS surround installations.
The DR2 follows the form factor of NTI’s second-generation Minstruments—a palm-sized unit with LCD, scroll wheel and Enter button, a set of quick-access buttons for Level, Frequency, and Waveform selections, a Power button that doubles as a display backlight switch, and a Mute button. Finally, there’s a button to set the resolution of the scroll wheel: 0.1 or 1 dB steps for level and frequency steps of 1/3, 1/6, 1/12 octave or 1 least significant digit. The case fits comfortably in one hand, and a soft, shockresistant jacket is provided, as well as a hand strap.
An RCA jack provides a transformerisolated 75-ohm S/PDIF or AES3-id (with the supplied RCA-BNC adapter) output. A male XLR supplies 110-ohm balanced AES/EBU or unbalanced AES3 with the supplied XLR-BNC adapter. A TOSLink optical output provides either stereo S/PDIF optical or an 8-channel ADAT Optical output. The normal output is stereo (two channels) but either channel can be switched off, or the polarity of one channel can be inverted with respect to the other.
XLR and RCA outputs span the full sample rate range from 32 to 192 kHz. The S/PDIF optical output goes to 96 kHz, though the ADAT output operates only at 44.1 and 48 kHz. I was pleasantly surprised to discover that the ADAT output can be used to test four channels of a 2X sample rate S/mux input. The optical output can be switched off, providing a bit of additional battery life.
A female XLR serves as a sync input should it be necessary or desired to synchronize the Digirator’s output with house sync. Normally the DR2’s internal low-jitter clock provides the data clock reference, but when a suitable signal is connected to the Sync Input, it is automatically detected, the Impedance menu pops up allowing you to select 75-ohm or 110-ohm termination or bridging, and the sync signal can then be selected as the clock source for the output data stream. Sync can be word clock, PAL or NTSC video black burst, DARS (AES3 without the audio) or an AES/EBU or coax S/PDIF audio stream.
While the DR2 is primarily a signal generator, it is also capable of verifying a channel or device under test for bit-accurate transmission as well as throughput delay (latency). The sync input connector doubles as the input for Transparency and I/O Delay testing. Firmware updates and audio test file loading and management are via a USB port.
The Digirator generates a variety of test signals including sine, pink and white noise, stepped or continuous linear or logarithmic sweeps (called “chirp”), a burst for measuring throughput delay (latency) and a polarity test waveform which works in conjunction with the NTI analyzers. In addition, Dolby and DTS nonlinear (encoded and compressed) PCM multichannel test signals in a variety of formats are available, offering channel identification, pink noise, tones, and a polarity test for surround systems.
The DL2 also functions as a WAV file player, allowing any waveform, test sequence, sound, or announcement that you can record to be output. Sound files, loaded via the USB port, are stored in folders in the DL2’s nonvolatile memory.
Selecting the FILE menu presents a list of available sound files. Because the memory size is limited and the non-linear test files are quite large, only a basic set is pre-loaded. Additional test files and some instrument sounds are supplied on a DVD. All test signals originate as 48 kHz WAV files, which get internally converted to the desired output sample rate. This limits the highest audio frequency to a tad below 24 kHz, a minor quibble, as the DR2 can’t check the full-range frequency response of a 96 kHz channel.
The Digirator offers control over the Pro/Consumer format, Non-Linear, and Emphasis channel status bits. Changing the Emphasis and Non-Linear flags doesn’t change the test signal, but it provides the ability to check the receiver’s response to those flags.
Up to 10 specific test setups (waveform, level, frequency, sweep time, etc.) can be stored as Configurations, which can be named (limited to eight characters, so you gotta be creative) via the USB port.
The obvious generator functions are very straightforward. Choose the test signal, connect the Digirator to the device under test, and go. The display and menus are intuitive, and the quick access buttons speed adjusting the signal parameters. For the more complex tests, the LCD displays what you need to see including the clock sync source, sample rate and, when running with external sync, the incoming clock frequency. Selecting SWEEP or CHIRP from the Waveform menu pops up a submenu for setting the starting and ending frequency, dwell or sweep time, and for selecting a single or continuous sweep. Selecting FILE displays the current folder and file name. The cursor and scroll wheel are used to navigate folders and select the file with the desired test signal.
The Transparency test is a simple go/no-go verification that all the audio data bits pass through the channel unchanged. Passing the test gives confidence that the channel will successfully transmit non-linear (encoded and data-compressed) data such as Dolby and DTS multichannel surround audio formats as well as conventional linear PCM audio. If the test fails, it’s time to get out a serious data analyzer such at NTI’s DL1 Digilyzer to locate the specific problem.
The manual is sketchy about what, specifically, is tested in the transparency test, and how to tell whether the test passed or not since it neither says PASS or FAIL. A change in sample rate or changed data bits is indicated by the generic “DATA CHANGED” message, which is indeed suspicious.
As long as both ends of the channel are accessible, Transparency can be checked with a single instrument. Two Digirators can be used on opposite ends of the channel, if necessary. In addition to end-to-end testing, a recorder can be tested by recording the Transparency test signal and playing it back into the DR2’s input for analysis.
I/O delay is another in-out test, which measures the time required for a pulse to pass through the AD-DA channel. The resolution for this test is 0.1 ms (also displayed as PAL or NTSC video frames). For checking latency through a DAW chain, a display in samples would be convenient.
There’s no official estimate of battery life, but NTI says that at trade shows, where it’s continuously on, they get four to eight hours from a set of alkaline cells. Auto shutoff and intelligent control of the display backlight can increase battery life. NiMH or Li-Ion rechargeable cells work fine, though you can’t recharge them in the unit. An optional AC power supply is available for bench use, however the manual recommends battery operation for the cleanest signals.
This is a handy gadget for testing digital inputs and transmission chains. It’s easy to use, can go anywhere, and can perform a range of useful tests. It’s a good tool for the maintenance shop or a broadcast facility, but probably overkill for the project studio. My only wish is that the audio frequency range extended to 40 kHz or above, but that would require a significant redesign.
Mike Rivers has a long list of engineering credits with the Smithsonian and is the author of the last Mackie HDR manual.