LOGIC CONTROL All logic control or "housekeeping" is performed by a single passive Programmable Logic Device (PLD). It was chosen over an active device for it's high speed and low noise operation and it's immunity to outside interference such as RF or static discharge. The PLD controls all Bus operations for the conversion circuits and incidental functions such as front panel indication and internal mute processing. The timing of this device has been mathematically structured so that the device itself is essentially "invisible" to the digital signals it must pass. Its contents are unique and proprietary. It is programmed by us and cannot be read or reprogrammed. The digital bus structure throughout is open architecture and is capable of 24 bit operation at double speed mode (96K) to accommodate all CD, DAT and DVD formats.

The Model 24MXL utilizes a unique micro-controller for signal analysis and reporting. The device contains high level code, designed by us, that enables it to operate silently. When an input is selected, or new data from the current input is detected, the micro-controller will configure the system to accommodate that signal, report the status on the front panel digital read-out, and then turn itself off. This feature allows the Model 24MXL to retain its excellent low noise characteristics. It will also enable us to be current with future digital audio formats with simple software upgrades. Software upgrades will be available at no charge to original owners.

CLOCK EXTRACTOR The circuit that regenerates the primary system clock of the source and provides the primary system clock for all other signals is called the "Clock Extractor". The system used in the Model 24MXL is fifth generation and is rapidly becoming accepted as the best. Sonically speaking, it is the most important part of the digital circuit. A poorly designed or improperly working clock extractor is the part of the circuit that can cause the jitter that will degrade the final output. The function of this circuit is so important to the correct operation of any digital audio device that it warrants some discussion here.

A digital audio source such as a CD transport is under the control of a primary high frequency crystal oscillator. The critical timing of all other digital clocks in the transport and hence the data they control is derived from this oscillator, it is often called the "System Clock". Since the processor is another link in the digital audio chain it too must eventually come under the control of this primary system clock. The system clock internal to the processor is just there for "idling" when no signal is connected. The problem is that the system clock from the transport is not sent to the processor via the standard digital audio interface formats. It is, instead, "implied" by the lower frequency clock that is sent down the wire or fiber ( often called the "bit clock" or "data clock"). The processor must then recreate an exact replica of the transport system clock for it's own use. The trick is not recreating the frequency of this clock but rather knowing when the lower clock signal it is derived from has been in some way corrupted by outside forces such as the transmission line problems, power supply problems, motor problems, (the list goes on). The point is that the clock extracting circuitry must make a "long term" analysis of the bit clock signal and regard the results of this analysis as being senior to any short term anomaly that is most certainly an error. The result is that any jitter from the transport is basically ignored while internal jitter is eliminated by the use of multiple power supply isolation and employing the very best temperature stabilized components available.

In electronics one second is long term. In a worst case scenario it will take the Audio Logic Model 24MXL less than one second to analyze the implied clock from the digital source. In most cases it will take 400 ms. This does not mean that optimal results are achieved in a half second. You will notice optimal timing characteristics after 10 to 30 minutes of operation.

DATA CONVERTER In all, there are four channels of data conversion which are combined in the differential output section to provide the derived stereo signal.

The data converter utilizes DELTA-SIGMA conversion techniques for the lower order bits and the more common "LADDER DAC" circuitry for the upper order bits. This approach takes advantage of the better qualities of both systems. The DELTA-SIGMA Dac delivering deep inner detail with the LADDER Dac delivering the authority and power needed to reproduce the lower mid and low frequency material. Laser trimming is employed to achieve the accuracy needed. Before actual conversion to the analog, the digital audio data is subjected to a 32x over-sampling linear interpolator/dither processor and then a 2x over-sampling sample and hold circuit. An additional bit position is added to accommodate the dither signal rather than have it "steal" a position from the sample word.

POWER SUPPLY The internal power supply is the commercial version of our laboratory supply. In all, there are four separate transformers used. This is the only way to achieve the low noise, distortion, and dynamic characteristics exhibited by the Model 24MXL. The high voltage B+ section utilizes a larger than necessary ( approx. 15X ) shielded transformer allowing for passive regulation. The filament supply is series regulated and isolated from all other sections of the power supply.

The digital supply for the control circuitry is series regulated with two stages of regulation providing for low impedance operation and ample heat distribution. Although heat is not a concern with the power supply some ventilation room should be provided.

The analog voltage for the quad DACs is totally isolated and is in three stages. This is to insure that the low level linearity an low intermodulation distortion characteristics of both DACs are preserved.

FRONT PANEL CONTROL & INDICATION INPUT SELECT

INPUT SELECT is done by the four push-buttons on the front panel. When first turned on the Model 24MXL will scan all the input ports and lock on to the first valid signal. After that, other inputs can be selected. All the inputs except for the Auxiliary Input are software compatible with the SPDIF and professional AES/EBU standard. The three pin XLR connector is hardware compatible with the AES/EBU transmission standard. The ATT ODL50 is hardware compatible with ODL50 transmitters and all optical transmitters operating between 810 nm and 850 nm. The auxiliary input is undedicated and can be configured (as an option) for any non-standard protocol, such as I2S or 13W3.

POLARITY SELECT Corrects for absolute polarity error in the digital domain (usually set to 0). If you believe that the digital material you are listening to was phase inverted in the mastering process, setting this switch to 180 will correct for this. There is no need to correct for a phase inversion of the actual digital signal sent by a transport or DAT. The input receiver in the Model 24MXL is very sophisticated and will detect this error and automatically correct for it. All bit periods will be perceived as occurring on the rising edge of the input signal.

REAR PANEL INTERFACE

ANALOG SECTION The line output connections are for left and right channel, RCA connector compatible. True balanced outputs via male XLR connectors are available as an option.

DIGITAL SECTION

The COAXIAL input connector is RCA compatible.

The AES/EBU connector is XLR compatible.

The (optional) OPTICAL connector is ODL50 compatible, requiring a male ST connector. We recommend glass optical fiber. Single mode optical fiber will function perfectly with the ODL50 receiver, provided it has been manufactured to the strictest tolerances.

NOTE: I2S and 13W3 ( all levels ) are available as options.