Post by thread:[OT] ISD sound chip questions

On Tue, 7 Apr 1998 13:28:14 +1200 Russell McMahon <spam_OUTapptechTakeThisOuTclear.net.nz> writes: >>Does anyone have a circuit for electricaly coupling the >sound output from an >>ISD sound chip to a small, one transister FM transmitter. I >have tried, but >>had to settle for acoustical coupling. >> > >I use USD chips extensively. Electrical coupling should be >extremely simple. Coupling from 1 speaker output pin to the >transmitter input with a capacitor (0.1uF probably OK) >should be enough. The one-transistor FM transmitters with the modulation coupled to the base of the transistor usually need only a small modulation signal. Too much, and the transmitter's frequency will go outside the range of the receiver. The raw output from the ISD chip is likely too much. In order to keep control the amount of modulation, a voltage divider should be used: ISD chip --| (one of the speaker pins) | 100k | \ / 10K pot \------||---- Transmitter / | | GND The coupling capacitor from the pot's wiper is probably already on the transmitter. Just remove the microphone and any biasing resistor for it and connect the ISD chip and potentiometer instead. Connect the transmitter ground to the ISD ground as well. Once a level has been found that produces acceptable results, the potentiometer and 100K resistor can be replaced with 2 fixed resistors. If the modulation can't be made strong enough with the pot turned all the way up, replace the 100K resistor with a smaller one. The speaker outputs are differential (one >is high when the other is low) and both are always above >ground so you don't need to use both to get a single ground >referenced output. Unfortuantely, on the ISD1000 (I don't know about the others), when the chip is not playing, both outputs are forced to ground. Using one for a single-ended output produces a "thump" when the chip starts and stops playing. A high quality system will probably need to use a differential amplifier or transformer to always take the difference between the two speaker pins. Since both are forced to ground at the same time, no "thump" will come from the differential converter. >>Also, any ways to copying the recordings from one ISD chip >to another? > >The only way that i am aware of is at the analog level - 1 >chip is set to play back and the other records. This is not >ideal but you do not have access to the inner workings of >the ic. This is generally the process. Of course, don't use the microphone amplifier and AGC on the destination chip. Put a gain adjustment between the two chips and adjust so both reordings are the same level. You should use the external clock feature to clock both chips at the same rate. The recordings should then end up at exactly the same address, regardless of variations in the internal clock generators. If preserving the signal frequency content is more important, use the internal clock generators. The clock rate can't be increased to speed up the process because the analog lowpass filters in the chips are not driven by the clock, so they would excessively filter a faster-sampled signal. ISD mentions they sell a box to replicate a chip's content either singly or in gang. I didn't see any details about the "programmer"'s availability or price. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]

>Unfortuantely, on the ISD1000 (I don't know about the others), when the >chip is not playing, both outputs are forced to ground. Using one for a >single-ended output produces a "thump" when the chip starts and stops >playing. This is true of all the chips. {Quote hidden}>>>Also, any ways to copying the recordings from one ISD chip >>to another? >> >>The only way that i am aware of is at the analog level - 1 >>chip is set to play back and the other records. This is not >>ideal but you do not have access to the inner workings of >>the ic. > >This is generally the process. Of course, don't use the microphone >amplifier and AGC on the destination chip. Put a gain adjustment between >the two chips and adjust so both reordings are the same level. You >should use the external clock feature to clock both chips at the same >rate. The recordings should then end up at exactly the same address, >regardless of variations in the internal clock generators. If preserving >the signal frequency content is more important, use the internal clock >generators. The clock rate can't be increased to speed up the process >because the analog lowpass filters in the chips are not driven by the >clock, so they would excessively filter a faster-sampled signal. ISD >mentions they sell a box to replicate a chip's content either singly or >in gang. I didn't see any details about the "programmer"'s availability >or price. There is however a subtle issue when copying chip to chip which may be a problem in some applications. When you stop recording, record continues to the end of the segment (50ms). There is also a delay between starting playback and the playback actually commencing. What this effectively means is that if you copy from chip to chip, the copied message will tend to be one segment longer than the original. I've found that you can do chip to chip copying at least 4 or 5 times between chips with no noticeable drop in quality. For the analogue interface, you need to use a differential amplifier to avoid the abovementioned clicks at the start/end. The cct below gives the correct level for chip-chip copy on ISD14xx parts: +>9V | |---[10K]-------| SP- --[10K]---[ - | | 1uF [ 1/2lm358 >--------[22K]----[]|--->AIN SP+ --[10K}---[ + | | | 1K 10K | | | GND GND +5V Note that the lm358 is assumed to be running from at least 9v, so the 5v reference is far enough from either rail for sufficient swing. For a 5V-only system, you should be able to connect the + input of the opamp to the junction of two 20K resistors, one to +5v, one to 0V ____ ____ _/ L_/ Mike Harrison / White Wing Logic / .....wwlKILLspam@spam@netcomuk.co.uk _/ L_/ _/ W_/ Hardware & Software design / PCB Design / Consultancy _/ W_/ /_W_/ Industrial / Computer Peripherals / Hazardous Area /_W_/