ADI-1 Inside

»Technical Information Index

Behind design, in front of technical issues and measurements

The ADI-1 is a small but extraordinary 20 bit AD- and DA-Converter, well designed, representing the actual engineer's approach. In this Tech Infopaper you will find much information, normally not presented in general overviews or the manual.

Analog to Digital

The analog inputs use Neutrik's combi jack, thus providing XLR- and 1/4" TRS jacks. The input stage is built in a servo balanced design. When used unbalanced it automatically corrects the gain by 6 dB. Thanks to this it doesn't matter how the inputs are connected, the level is always as it should be. Changing the working level between +4 dBu and -10 dBV is done with a special damping method already in this stage. Although there is only limited supply voltage of +/-5 Volt available, this technique allows a maximum input level of +20 dBu.

The servo balanced input stage is followed by a low noise amplifier stage, providing a variable gain between 0 dB and +20 dB. The adjustable hi-precision resistor is free from DC offsets, so no noise or crackle will come up now or later. Both stages are realized using the low noise and low distortion 4580. This allows us to provide a wide range of input level adaption (31,8 dB) without lowering the sound quality of the audio signal finally reaching the AD-Converter chip (a CS5335.)

Fig. 1 shows the converter at work. A pure signal without noticeable distortion, and a very low noise floor. Fed to the analog input was a 1 kHz sine, with a level that resulted in -60 dBFS in the digital domain.

Further Specs:
THD+N
 -89 dB or 0,003%

Signal to Noise (SNR)
Gain 0 dB:
100 dB, 102 dBA
Gain +20 dB:
89 dB, 91 dBA


Fig. 1

Just compare this to the measurement made at a normal consumer DAT. Because most users already own one these devices are often used as AD-Converters. In this case not only the balanced input and the professional input level adaption is missing. Of course a 16 bit converter won't reach the specs and quality of a 20 bit chip. So it's no wonder that the diagram shows distortion and a 12 dB higher noise floor! That's why many engineers claim the 20 bit chips to be perfect 16 bit converters.

The ADI-1's frequency response misses any noticeable deviation, as will be shown later. But don't be surprised: thanks to multiple oversampling the frequency response is straight not only in our devices, but in nearly all devices available in the market. Times have changed, and an engineer has to make big mistakes to introduce any deviation here. This can be seen in magazines when testing such digital devices like DATs...you won't see a difference...

Fig. 2

The digital output stage is made out of a CS8402. The chip sends its digital signal to a coaxial (Cinch) and an optical (TOSLINK) output in SPDIF format. The so called 'Channel Status' is fixed to Consumer format without copy protection, so ADI-1 is compatible to most digital appliances. Even professional devices with AES/EBU inputs can often be connected to the ADI-1's output if a cable adapter is used.

Controller

The ADI-1 is very easy to operate thanks to a special programmed controller handling complex multiple processes inside the ADI-1. The integrated circuit (for insiders: a PLD) controls the sample frequency of the AD-Converter, the change to another sf, the display of the sf at the DA-Converter's input, the reset of the converter chips and their DC-adjustment, and much more. Most work has to be done when decoding the level information from the AD-Converters. Converting this data stream into a digital, 100% error free LED Peakmeter with reliable Over detection needs much resources. But it's the only way the ADI-1 will be usable without any further equipment, enabling you to make records with highest levels and lowest noise, without any clipping and distortion.

DA

Crystal's well known 8412 is used as digital receiver. A switch on the back of the ADI-1 decides whether the coaxial (Cinch) or optical (TOSLINK) input is active. As all Channel Status information is ignored, no copy protection, consumer/professional or even wrong information will prevent the ADI-1 from accepting the input signal. In other words: it works in every situation! The PLD decodes the already decoded information from the 8412 a second time, providing not only three LED's for the actual sample rate but also an error LED in case of a faulty or damaged signal.

The output signal from the DA-Converter chip (AKM4320) is fed directly and unchanged into the servo balanced output stage. This stage is adjusted internally to work full symmetrical, and provides - like the input stage - an automatic gain correction of 6 dB when using balanced or unbalanced connectors. The used 4580 is not only low noise and low distortion, but also a very good line driver, so the analog output signal is available with low impedance and noise free at the XLR- and TRS jacks.

Fig.3 shows a digital generated 1 kHz sine at -10 dBFS, measured at the analog ouput of the ADI-1. When comparing this to the measurement of the AD-Converter you will notice some distortions and a slightly higher noise. This is correct and normal. The quality of ADC's is much higher as that of DAC's since years, when comparing in the same bit class.

Further Specs:
THD+N
 -88 dB or 0,003%

Signal to Noise (SNR)
96 dB, 98 dBA unmuted
106 dB, 108 dBA muted


Fig. 3
   
   
To see the real advantage of the ADI-1's 20 bit converter just compare it to a usual 16 bit consumer DAT. The diagram in Fig. 4 shows a very unstable noisefloor plus a wide range of added harmonics (of which distortion is made of.) It is obvious that the advantages of a 20 bit converter are also present, useful and needed when working with material in 16 bit resolution!

Fig. 4

Perhaps you noticed that we claimed ADCs to be always better than equivalent DACs. On the other hand our ADI-1 reaches 102 dBA dynamic on AD, but tremendous 108 dBA on DA side.
 
Many DA-Converter chips include a mute circuit. As long as a signal is present, the real noise of the DA-Converter is present and limits the dynamic on somewhat under 100 dB. But at complete silence (feeding digital zero), a special mute circuit acts like a noise gate and mutes the analog outputs of the chip. This method leads to much higher values up to 110 dB, not representing the real resolution and signal to noise ratio as expected by the costumer. RME therefore presents both values (muted/unmuted.)

So the resolution of modern 20 bit converters is limited at low levels to 'real' 16 bit. Although the converters really have 20 bit resolution (can be proven with a hi-resolution FFT), all signals below the 16 bit dynamic range will vanish into the noise floor for the human ear. Therefore a 20 bit converter is a perfect 16 bit converter, but far away from its theoretical dynamic (120 dB.)

Fig. 5 finally shows the frequency response of AD and DA in one picture. As with all measurements the ADI-1 was connected as you would connect it, at the outside jacks. As said before the response is neither spectacular nor exciting. But on the other hand it is good to know that the ADI-1 is perfect in this situation too. 


Fig. 5

Power supply

The design of the power supply has to be done with regard to costs, mechanical issues and electrical basics. Because of the size of the ADI-1 an internal transformer requires an expensive magnetic shielding, or results in an unwanted hum. A design without an earth wire (to avoid hum loops) would also be nearly impossible. An external power supply needs no magnetic shielding, causes no hum inside the ADI-1 and is much cheaper. As such an external transformer does not need an earth or ground contact, there also will be no hum loop when using the ADI-1.
 
We also recommend the use of optical cables. Thus the ADI-1 is the perfect, complete ground isolated front end for RME's digital in/out cards. At no time recordings and playbacks using the PC sounded better! And if anyone tells you that optical cable introduce jitter and are very delicate to handle, and it's better to use wired connections: forget it! This fairytale from the early days of digital audio is proven to be simply a fairytale, nothing else.

Back to the power supply: we use a simple 12 V / 850 mA AC adapter. Inside the ADI-1 12 Volt AC is converted to positive and negative DC. There is one disadvantage of external AC adapters: the grounding design on the printed board is extremely delicate, that's why such devices often suffer from low but audible hum. The engineer has to design the layout very carefully (most times it needs several layouts to get the optimal result.) As the above FFT Analysis and the numerical values prove we succeeded in a total hum free design for the ADI-1. The small difference in the measurement values between RMS unweighted and A-weighted are caused only by the different noise sensitivity in the higher frequency range. Hum is not present, else we wouldn't have started the production of this device...

Notes on measurements

RME does all measurements using professional audio analyzing tools and standardized methods of measurement. What's more the measurements are not done under strange or special conditions, but so as the device would be used in reality, thus not bothering you with values anyone can only dream of (and nobody knows where they come from.) Some companies reduced their efforts in examining their own devices to a reprint of the highest values published by the manufacturer's of the corresponding chips. These values can't be reached, because in the final device there is always additional electronics around the chip. And it is no secret that the values published in the chip's data sheet are often goals the manufacturer's want to reach (often only reached in later chip revisions or never.)

At RME you don't need to read tests of independent magazines. We offer real and true measurements at no extra cost!

Analog and digital measurements are made using Neutrik's A2D with its Windows software AS04 (www.neutrik.com.) This 'Audio Test and Service System' is capable not only of RMS unweighted and weighted measurements, like level, noise, distortion, or the above frequency response. Thanks to its easy to understand and fast user interface, the highly precise measurement results and its flexible input and output interface the AD2 is a must for every hardware developer.

When working in the digital domain we also use a tremendous Windows software: HpW Works is a FFT analyzer with incredible resolution and accuracy (see the above FFT diagrams.) The included software generator provides many different test signals in 64 bit accuracy.

Both systems are reference at well known and respected magazines, like ELRAD or c't (www.heise.de/ct.)

Glossary

16 bit: Equals a resolution of 65536 level steps or theoretical 96 dB dynamic range.
20 bit: Equals a resolution of 1048580 level steps or theoretical 120 dB dynamic range.
+20 dB: Logarithmic ratio. Equals 10 times amplification.
AES/EBU: Professional, balanced digital interface using XLR connectors.
dBA: RMS measurement using an A-filter. Less sensitivity at low and high frequencies.
dBFS: dB Full Scale. Logarithmic level value relative to digital full level.
FFT-Analysis: Fast Fourier Transformation. Dividing any signal in its spectral components.
Jitter: Fluctuations of a signal in time domain.
RMS unweighted: RMS measurement using an audio bandpass (22 Hz to 22 kHz.)
Sample frequency: Number of probes taken from the audio signal per second.
Servo balanced: Circuit design with automatic gain correction in balanced and unbalanced mode.
SPDIF: Sony/Philips-Digital Interface, consumer version of the digital audio signal.
Signal to Noise: Ratio of maximum signal level to noisefloor without a signal present (SNR.)
THD+N: RMS measurement of all harmonics plus noise relative to the sum of all signals together.

Copyright © Matthias Carstens, 1998.

All entries in this Tech Infopaper have been thoroughly checked, however no guarantee for correctness can be given. RME cannot be held responsible for any misleading or incorrect information provided throughout this document. Lending or copying any part of the complete document or its contents is only possible with the written permission from RME.
 

Home    News    Audio Converters    Sound Cards    MADI Series     DIGICheck   Mic Preamps
Accessories    Support    RME Newsgroup    Company Info    Purchasing    Downloads    Links

Copyright 2002 RME. All rights reserved. RME is a registered trademark.
This website contains names and marks of other companies.