A/D Converters - part three (Field Test)

by Bob Hodas and Paul Stubblebine

Once again we find ourselves at Rocket Lab Mastering Studio in San Francisco. The three new converters we listened to were the Neve HRC-1 at 20 bits dithered, the Sony DAD-A2000 at 16 bits, and the Wadia WA4000 at 20 bits. We also listened to the Apogee AD-500 with the current modifications.

Hopefully you are all familiar with the drill by now. If not, please refer to Mix, May '91 and March '92 for a complete description of recording and listening procedures.

Apogee A full description can be found in our Mix, March '92 article. The new modifications claim extended deep bass and tighter bass in general. Imaging also is said to be improved.

Neve The HRC-1 is a two rackspace unit that houses both A/D and D/A converters. Neve offers a great deal of internal routing options as well as bit rates of 16, 18, and 20 bits. The converters are Neve's proprietary design. The HRC-1 only converts the AES/EBU format. No SDIF-2 or S/P-DIF is available. Internal switching allows selection of 110 or 230 volts AC. Input and output levels are also internally selectable using jumpers. Neve claims pin 2 hot for this unit but we found that it was actually pin 3 hot on the A/D side and pin 2 hot on the D/A side. This is an obvious error that they should have noticed in quality control. The front panel is logically laid out in seven sections. The Synchronization Source panel contains two switches and an LED. A three position sync source switch selects the digital audio XLR, the digital sync XLR, or the internal clock as the master clock. The sample frequency selection switch selects either 44.1kHz or 48kHz sampling. Automatic sampling selection occurs when in the digital audio XLR mode. A reference lock LED indicates lock to the digital sync XLR. The Phase Lock Loop section houses a switch selecting chase or normal modes. The chase mode is used for high jitter clock sources while the normal mode works with low jitter sources. If the switch is in normal but there is too much jitter to lock, the unit automatically selects chase. The unit locked LED indicates when the PLL has locked to its source. The analog outputs always have 20-bit dither applied but dither for the digital output is selectable in the Re-Dither section. Dither on or off is selected via a two position switch. A second switch selects whether you want 16, 18, or 20 bit dither. This means that you can dither in the digital domain, a feature unique among A/D converters. The De-Emphasis switch and LED select de-emphasis on the digital input audio. Two routing switches select internal signal routing. You may choose "1" Analog in to Digital out-Digital in to Analog out, "2" Analog in to Analog out-Digital in to Digital out, "3" Analog in to Analog and Digital out, and "4" Digital in to Analog and Digital out. Seven Digital Input Status LEDs indicate the following: Lock indicates lock to a digital input source clock. Sync shows that the digital in is synchronized to the PLL clock within +/- 25% of the sample period. PAR ERR indicates parity error. CRC ERR indicates corectable digital errors. Non-Audio indicates that the non-audio bit mode is set, thus muting the analog out fed from the digital input. Invalid A indicates an invalid AES 'validity" flag for channel A. Invalid B indicates the same for channel B. Input Threshold LEDs warn of signal -1.16dBFS for both the analog and digital inputs. Lastly is the power switch and LED indicator. The rear panel contains all XLR connectors for analog in and out, digital in and out, and digital sync in. Along with a very quiet fan, a standard IEC power cord receptacle is also present. The HRC-1 retails for $9,785.00

Sony The DAD-A2000 /A2000R consists of a main frame and an assortment of cards. The DAF-2000 frame is a two rack space unit containing a power supply, video clock board, clock board, and room for four cards. Cards available are a DABK 2001 A/D converter, and a DABK-2002 D/A converter. The A/D operates at 16 bits utilizing 1 bit Delta Sigma technology while the D/A uses a digital filter of 16-bit input and 18-bit output with 8 times oversampling. Cards may be mixed and matched allowing for up to eight channels of audio. This unit mat be connected to a digital audio tape recorder or a digital VTR. It converts only AES/EBU. The front panel is actually a hinged door that swings open to reveal the various cards. It has a smoked plastic window that allows one to see significant LEDs and an indicator that shows power on or various problems analyzed by a self diagnostic logic. Inside the front panel door is the power switch and a headphone jack with volume pot. This jack receives the same signal as the back panel Monitor Outs. The CK board has two functions. The Sync Clock button selects for LEDs indicating internal clock, external video sync, external digital sync, and word sync. The Sampling Frequency button selects LEDs indicating 48, 44.1 or 44.056kHz. If external clock is supplied then the unit will follow 44.1kHz +/- 12.5% and 48kHz +4%, -12.5%. The D/A can operate in a range from 38-54kHz. The VCK board has four LEDs indicating Frame Frequency of an external video input signal. The D/A board contains a Pre-Emphasis On/Off switch, 5 segment stereo LED Input Level Meters with Input Trim Pots, and an Input Monitor Button that routes the analog signal to the headphone jack and monitor outs on the back panel. The A/D board has LED indicators for De-Emphasis on channel 1 and 2, Mono signal detection, and AES/EBU format error detection. There is a five segment stereo LED Output Level Meter with analog Output Level Trim Pots. The Monitor button routes the analog signal to the headphone jack and monitor outs on the back panel. The rear panel contains space for four converter backpanels. Permanently residing is the AC receptacle, and a very noisy fan. The XLR Monitor Outputs can be used to monitor signals from the A/D or D/A cards. The user may select pin two or three as hot with a jumper cable. Two BNC connectors and 75 ohm termination switch are provided for a Reference Video Input and loopthrough. Two more BNCs and switch are for Word Sync Input and configured the same as above. Retail price for a DAF2000 frame is $5,000.00. The A/D card is $2,000.00 and a D/A card is $1,500.00

Wadia The Wadia WA4000 is a single rack space utilizing the Ultra Analog ADC20048A 20 bit converter chip. Previously a consumer audio company, Wadia seeks to bring "audiophile" engineering into the pro world. They claim proprietary designs for ROM upgradability and enclosure detuning. They also claim an extremely accurate crystal clock at 2PPM. Formats supported are AES/EBU, SDIF-2, and S/P-DIF. The front panel is simple and elegant. A momentary button selects a sampling rate of 48kHz or 44.1kHz, with an LED for each. A Mode button selects four bit rates and polarity. These modes are 20 bits, 20 bits polarity inverted, 16 bits dithered, and 16 bits dithered polarity inverted. Inverted polarity is shown with an LED and 16 bit status lights the two -20dB level indicators. Another button selects between Internal and External sync. LED indicators designate which is active and a third LED shows Lock. There are seven level LEDs for each channel and an over LED as well. The rear panel houses the AC receptacle and fuse along with a variety of digital connector options. There is a true 75 ohm BNC coax connector for S/P-DIF and two independent XLR AES/EBU outputs. There is also an XLR AES/EBU External Sync input. SDIF-2 connectors are BNC for Word Clock in and out and Channel 1 and 2. Lastly is a glass fiber optical output which is not compatible with the current plastic fiber optic interface. Glass is capable of longer runs but at this time we know of no pro manufacturers who support this interface. The two analog inputs also have recessed level control pots. These are not even full turn pots. For .1dB accuracy, multi-turn pots would do a better job. Retail price of the WA4000 is $4,500.00

LISTENING TESTS The first piece was "Waiting Song", a current folk-pop release by Barbara Higbie. Instrumentation was piano, vocal, electric bass, drums, congas, acoustic guitar, and percussion.

Apogee Bass has good punch and the track a good deal of life. High frequency air is good but sibilance seems a bit exaggerated and hard. Image is good. Moving harmonics on held piano chords in the intro provided a good test of low level resolution. The Apogee resolution was good but some warmth was missing. Vocal a bit darker. The pulse or rhythmic drive of the track was well preserved. Neve Life or presence is missing. Loss of warmth and deep bass. Lower harmonics on piano not strong. Image is good. Vocal dark. Sibilants ragged. Sony Very good image. Deep bass present but some punch is gone. Reverb and low level resolution is good. Instruments a little smudged together. Slight air loss. Sibilants have a little extra but not harsh. Good passing harmonics on piano. Good life. Vocal loses a little bit of body. Wadia Lively. Image good. Pulse is good but deep bass and warmth are lacking. Passing harmonics all right. Upper air gone and sibilants a bit gritty. Vocal gets a slight megaphone quality. Bass and midrange seems disconnected.

The second piece was a mono narrator reading copy from a magazine. Half of this selection included some background noise that added dimension and demonstrated low level resolution quite well.

Apogee Tonal balance lightened. Close, but no low bass. Good low level resolution. Neve One step removed, just that little bit of presence missing. Low level information discernible but less resolved. Tonal change. Sony Closest. Vocal loses some definition. Good low level resolution. Good pops Wadia Once again a slight megaphone quality. Good background noise. Pops good.

Next came the Slavyanka Chorus, a choir of twenty-five male voices, recorded with a Blumlein mic technique in the chapel of Saint Vincent's School, San Rafael.

Apogee Good blend. Good life. Tonal balance a little lighter. Acoustic depth remains. Slightly strained on loudest passage. Neve Flat, gray. Life gone, no depth and reverb less. Warmth lost. Sony Mix clearer but hall sound is reduced, depth lost. Some harshness on loud passages. Wadia Good dynamics. Good sense of hall at low levels but fades out as music gets loud. No strain on peaks.

So, do we have a clear winner? As you you see, we cannot say that any one box fills all the bills. It is hard to give a concrete score but here is a general roundup of our impressions. On Barbara the Apogee and Sony seemed to best represent the music. The spoken word found the Wadia on top with Sony and Apogee in the second ranking. With the chorus, the Apogee felt closest with Wadia and Sony close behind. We can say that the Neve was generally dissapointing in all categories.

We are still searching for the near perfect representation of life. We can't say we're satisfied yet. The industry still has a way to go and new boxes are in the works. We will continue listening so look for part four in a few months. Among the new entries will be Lexicon, Sony's new 20 bit card and who knows what.