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Build this 20ms delay board to add to your surround sound unit or use it to enhance musical instruments. It uses the latest digital conversion and memory storage techniques to pro­vide quality sound. Only one integrated circuit is required, plus a handful of passive components. By JOHN CLARKE A 20ms Delay For Surround Sound Decoders I F YOU HAVE BUILT a low cost surround sound decoder, you won’t be getting the best sound effect unless it has a delay for the rear channel. Now you can fix this drawback by adding our 20ms delay board. A typical lounge room with a surround sound setup will have the front, left, right and centre channel loudspeakers located well in front of the lounge and adjacent to the TV set. The rear loudspeakers will be directly behind the listener. Because of this, the sound from the rear will arrive at the listeners ears before that from the front loudspeakers. When this occurs, the sound from the surround loudspeakers will tend to dominate the perceived direction of the sound field. However, if a delay is added to the rear channel, its sound will arrive later than from the front and so the sound field will be correctly perceived by the 60  Silicon Chip ear. The normal delay time required is 20ms and this will cater for most lounge rooms. For public address use, it is also sometimes an advantage to add a delay to the sound fed to the loudspeakers at the rear of a hall compared to those at the front. For listeners at the rear of the hall, the sound from the loudspeakers near to them will normally arrive before the sound from the loudspeakers at the front. The result will be an echo that can cause considerable difficulty in under­standing the person speaking. By adding a delay to the rear speakers, Performance Delay 20ms (fixed) Gain Unity Frequency Response -1dB at 10Hz, -3dB at 7kHz Maximum Input 1.2V RMS Input Impedance 20kΩ Output Impedance 1kΩ Harmonic Distortion 0.3% at 1kHz and 300mV RMS (see graph) Signal To Noise Ratio With respect to 1V RMS: 97dB unweighted (20Hz-20kHz); 101dB A-weighted Fig.1: the M65830 IC converts in the incoming signal to digital form, stores it in memory, reads it out again and converts it back to an analog signal. The delay is a function of the size of the internal RAM and the clock speed, as set by the oscillator. this echo effect can be considerably reduced. A 20ms delay represents the time that it takes sound to travel 6.7 metres. Several delay units could be connected in series to increase the delay if necessary or, better still, you can alter the circuit components to obtain a 40ms delay. The circuit is based on the Mitsu­ bishi M65830P digital delay IC. This works by first converting the incoming analog signal to a digital format which is then clocked into memory. This digital signal is then clocked out at the end of the delay period and converted back to analog form. In addition, the M65830P contains AUDIO PRECISION THD-FRQ THD+N(%) vs FREQ(Hz) 5 21 DEC 95 13:16:35 1 0.1 0.010 10 100 1k 5k Fig.2: this graph plots the harmonic distortion for the 20ms Delay Circuit. The ripple evident in the curve is an artefact of the A-D (analog-to-digital) and D-A processes but is not audible. several op amps so that input and output filters can be added to the circuit without using addi­tional ICs. Circuit details The circuit is shown in Fig.1. The input signal is coupled to an inverting op amp input at pin 23 via a low pass filter comprising C1, C2, R1, R2 and R3. This rolls off signals above about 8.5kHz to prevent high frequencies affecting the following digital conversion which can cause spurious effects in the out­put. Capacitor C6 and resistor R7 control the rate of delta modulation which is the type of analog to digital conversion used in IC1. Similarly, C5 controls the digital to analog conversion output signal appearing at pin 15. This output is applied to a 7kHz filter comprising resistors R4, R5 and R6 and capacitors C4 and C5. The output of the filter op amp at pin 13 is AC-coupled via a 10µF capacitor. The circuit can be powered from a DC rail from 9-25V, although we have shown +12V as the supply input on Fig.1. Three-terminal regulator REG1 provides +5V to the IC’s Vcc and Vdd pins (1 & 24). Options Some readers may have an application which requires a longer delay time than 20ms or may desire a frequency March 1996  61 18kΩ; C1 & C3 should be 560pF and C2 & C4, 150pF. Note that using 15kHz filters will lead to a slight increase in residual noise and distortion. The harmonic distortion char­ ac­ter­­istic for the 20ms circuit, with 7kHz filters, is shown in Fig.2. Construction Fig.3: follow this parts layout when building the PC board. An IC socket is optional. Fig.4: this is the full-size artwork for the PC board. Check the etched board carefully before installing the parts. response above 7kHz. If you want a 40ms delay, the crystal must be 1MHz (instead of 2MHz), C5 & C6 should be .022µF and R7 should be 82Ω. To obtain a frequency response to 15kHz, the following components should be changed: R1, R2, R4 & R5 should be 39kΩ; R3 & R6 should be All of the delay circuit components are assembled onto a PC board coded 01401961 which measures 63 x 60mm. It is designed to fit horizontally into a plastic case measuring 130 x 67 x 32mm. This case is optional – we envisage many constructors will install the board into existing equipment (presumably the same case as the surround sound decoder). The component overlay is shown in Fig.3. Begin construction by checking the PC board for shorted or broken tracks. Repair any faults before assembling the compon­ents. Start with the PC stakes for all the input and output terminals. Next, the links can be installed, followed by the resistors. Use the accompanying table to assist you in selecting the correct colour code. Once the resistors are in, the IC and the capacitors can be installed. Make sure that the IC and the electrolytic capacitors are correctly oriented. Finally, install the 3-terminal regulator. It is oriented so that its tab faces the adjacent 10µF capacitor. Testing To test the unit, apply power to the DC input and check with your multimeter that the regulator output is close to +5V. PARTS LIST 1 PC board coded 01401961, 63 x 60mm 1 plastic case 130 x 67 x 32mm (optional) 1 2MHz crystal (X1) 6 PC stakes 1 50mm length of 0.8mm tinned copper wire Semiconductors 1 M65830P delay (IC1) 1 7805T 5V 3-terminal regulator (REG1) Capacitors 1 100mF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 4 10µF 16VW PC electrolytic 3 0.1µF MKT polyester 2 .068µF MKT polyester 1 .0027µF MKT polyester 1 .0022µF MKT polyester 1 680pF ceramic 1 560pF ceramic 2 100pF ceramic Resistors (0.25W, 1%) 1 1MΩ 2 10kΩ 1 100kΩ 1 30Ω 4 22kΩ If you have access to an audio signal generator, you can use it to check the circuit operation. Note that the gain of the circuit from input to output is unity. A dual trace oscilloscope can be used to verify the delay between input and output. At 25Hz, the two signals should be 180° apart, while at 50Hz they will appear in-phase again. Final testing can be made after installation in your equip­ment. Note that the delay unit will only handle signals up to 1.2V RMS. For higher level signals, an input attenuator will SC be required. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ 62  Silicon Chip No. 1 1 4 2 1 Value 1MΩ 100kΩ 22kΩ 10kΩ 30Ω 4-Band Code (1%) brown black green brown brown black yellow brown red red orange brown brown black orange brown orange black black brown 5-Band Code (1%) brown black black yellow brown brown black black orange brown red red black red brown brown black black red brown orange black black gold brown