Know-how

Better than the Microphone

Being its first user, Mr. Günther Pauler employed the 28-bit converter made by Stage Tec. He was able to achieve remarkably transparent recordings in the »Kerk Van de Doopsgezinde Gemeente« in Haarlem, Netherlands; this church is renowned for its excellent acoustics. Our photo shows a session in the church, with the Spirit of Gambo ensemble recording their new CD – supported by a True Match RMC unit by Stage Tec.

Imagine this recording engineer had a microphone preamp with more headroom than the microphone itself; no level pre-adjustment would be necessary. Further, it would be nice to have an A/D converter with a dynamic range greater than any mic – we could virtually forget about level issues during recording sessions and could »fix it in the mix« later. What a wonderful dream. Sometimes, however, dreams come true: Stage Tec's new 28-bit converter offers just that!

Hardly Input Noise

The combination of the input and converter stages of the new system creates an input noise of –134 dBu – roughly the same as the input noise of a 60-ohm resistor. Now what about the microphone? With a sample impedance of 200 ohm, its noise performance is limited to a theoretical optimum of –128 dBu. Thus, the microphone noise »masks« the converter noise.

In a combination of a microphone and our converter, it is the microphone that determines the bottom end of the dynamic range. Now let's look at the top end. The new converter handles input levels of up to +22 dBu – much more than the highest microphone-output level. This means that it is practically impossible to overload the converter input. To achieve a total 150 dB of dynamics, Stage Tec developed a new conversion method called TrueMatch that meanwhile was issued a patent for.

The previous version of NEXUS converter boards already used a 24-bit TrueMatch; however, their resolution was limited to 22-bit words. The new generation of converters has a resolution of 26.5 bits that is enhanced to 28 bits. The user benefits from this enhancement in two ways as not only quantization noise is avoided but the input noise can be used for »natural dithering«. With 26.5 bits, the quantization noise is equal to the converter noise, resulting in a total deterioration of 3 dB. Increasing the resolution to 28 bits reduces the quantization noise to about 10 dB below the converter noise; thus, quantization noise is practically not perceivable anymore. Moreover, the microphone noise must still be added – and this is a minimum of 6 dB above converter noise.

Today, however, there is no commercial recording system capable of storing a 28-bit signal. So, what use has a converter with a resolution such high? When recording on a 16-bit medium, there is actually little improvement: Although the converter's high linearity can certainly be perceived, clipping may occur. The situation improves where a data compressor is used to process a 20-bit signal in a way that it can be stored on a 16-bit medium, providing you with a dynamic range of 120 dB. Even if we deduct a 20-dB headroom, we still can utilize 100 dB. This assumes that the converter is properly lined up as it is used as microphone preamp at the same time. Anyway, pre-amplification can be adjusted between 0…70 dB – not too bad.

This adjustment can be omitted if a true 24-bit recording medium is used. In this case, gain is always adjusted to +15 dB for a dynamic microphone and 0 dB for a condenser microphone, and everything that the microphone delivers will be recorded. This may be of special interest for high-end mastering applications and in DVD production where 24-bit recording has become a standard. Finally, in combination with CANTUS, the full 28-bit sensation is guaranteed. If there is clipping, it was the microphone, or you have unfaithfully inserted an old analog microphone preamp into the chain.

Versions

The new 28-bit converter is available in two versions: a module board for the NEXUS system and a stand-alone version. The 4-channel NEXUS board is called XMAD (NeXus Microphone A/D converter). In conjunction with CANTUS, XMAD shows its superiority: the full 28-bit word is delivered to CANTUS via a NEXUS direct out, and CANTUS is capable of processing each and every bit! Processing an AES/EBU signal is a different thing: the standard limits the signal width to 24 bits.

The other version – the TrueMatch RMC stand-alone converter – features a 24-bit resolution, too. It supports up to 24 input channels and provides a high-quality front end for recording and mastering studios.