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MIXING TRICKS

Simon Goodwin explains how to mix computer and CD sound on any Amiga

Introduction

CD drives are deservedly popular Amiga expansions, yet they've always been an optional extra, except on the ill-marketed CDTV and CD32. While it's not difficult to add a CD onto an existing computer, it's trickier to get the full benefit, integrating CD audio as well as data with native Amiga sound. With 16 bit sound cards on A1200s as well as big-box Amigas, even more connections need to be made.

This article provides the missing links, giving tested, practical advice on audio mixing for all systems. The adapters described use cheap, readily-available parts. You don't even need to solder them together.

Amiga Audio

Stereo audio was an Amiga strength long before the word "multimedia" became commonplace. Every Amiga has two phono sockets (also known as RCA or CINCH) carrying line level signals to a stereo monitor or amplifier. These connectors are also used on HiFi stereo cassette decks, tuners, and audio CD players. If you're used to mono sound through a TV modulator, you'll be amazed by the improvement once your Amiga is re-routed through a decent "separates" HiFi.

There's no shortage of SCSI and IDE/ATAPI interfaces, drives and drivers, to connect a CD ROM and load software. Virtually all CD ROM drives will also replay audio disks, as they are derived from audio players and include the same 16 bit stereo digital to analogue converters, often integrated into their control circuits.

The challenge is listening to CD audio as well as the output from the Amiga. Many modern games expect a combination, running background mood music from CD, augmented by samples in memory for spot effects. It's often nice to be able to listen to CD audio without missing sonic cues from your Amiga software.

Hardware

Most CDROM drives have a headphone output on the front, with a volume control. The shaft of the plug is the earth connection, with left and right signals on tip and ring. This is conveniently accessible, but lower in quality than the line output at the back of the drive. The headphone amplifier introduces noise and boosts interference from the drive motors, causing background buzz and zipping noises. It can also overload HiFi inputs, with potentially expensive consequences.

Many beginners switch from CD to Amiga output by swapping leads between this and the Amiga's phono outputs, but there are more convenient, better-sounding approaches. It's preferable to use the "line output" connector at the back of the drive, converting signals to standard format and level before mixing.

CD Audio cables have three or four pole connectors. NEC, Sony, Mitsumi and Matsushita have their own "standards", varying even among drives from the same manufacturer. The illustrations show the back panels of typical ATAPI and SCSI drives, from Tatung and NEC.

Some drives have two digital audio output pins, opposite the power input. These combine both channels in a serial data stream at over a megabit per second, like external gear using SP/DIF (domestic, phono) or AES/EBU (professional, XLR) digital audio standards. Current Amiga expansions lack these facilities, so we'll stick with analogue for now.

Solder Bath

I recently rescued a drive from Bath with wires soldered directly onto the output pins. This is dodgy - there's too much risk of damage and short-circuits. Failing the manufacturer's cable, try a standard 0.1" pitch three or four way socket, or individual push-fit crimp connectors.

If there are just three pins you can be confident that the middle one is the ground, with left and right either side, matching A3000T and A4000 connectors. These connectors are arranged LGR, or RGL from the other side; suck it and see, no harm can result.

If there are four pins at the CD end, one is redundant. You need to work out which of two configurations your drive uses. Recent production favours LGGR (Left Ground Ground Right) connections. This Sony scheme needs a four way connector. When the signals are arranged RGLG (Right Ground Left Ground) you can use just the first three and skip the second earth. The full connector has the advantage that left and right can't be swapped inadvertently.

Aim to get a lead with the drive, and adapt the other end to suit your setup. Failing that, CPC in Preston stock half a dozen types. Manufacturers can use any colour code they fancy, but the Left lead is typically white, Right Red and Ground Black.

Mixing

External cased drives should have standard stereo jack or phono sockets, but generally still need a mixer to combine and balance the levels. There's leeway in making the connection between line output and amplifier. The outputs have a low impedance, and might supply up to a milliAmp of current - plenty for line inputs with nominally 10KOhms input impedance. In general, a low impedance output drives higher impedance inputs.



This active mixer design from Aminet boosts line outputs

If you join several outputs to one input the outputs interact, as the signal favours the low impedance path to earth - through another, competing output circuit - rather than the intended input. Our circuits discourage this by introducing resistors between the outputs. These limit the current, effectively preventing outputs strangling one another, and can balance signal levels, so Amiga audio need not drown out the CD output, or vice versa.



Six resistors form a simple two input stereo mixer

AlfaData's simplest possible arrangement uses a pair of 5.6K resistors, in series with the CD outputs. This works, but risks mismatched volume levels. Our first circuit uses a couple of potentiometers. The input signals feed in from each side and out through the slider, so moving the slider determines the balance between CD and Amiga audio, or any other pair of signals.

Any pair of linear potentiometers with a value of a few thousand Ohms is suitable. I uses 10K presets which cost about 20 pence each, soldered to a piece of Veroboard. Preset parts, designed to be set and left, are cheap and can be adjusted empirically to suit your equipment.



Build an audio mixer from two 10K linear potentiometers

Ratios

If soldering leaves you cold, fixed resistors are cheaper still and can be screwed, with the cables, to a plastic connector block. Use pairs of resistors for left and right, picking values to match the levels. You don't need to be exact; resistors are usually supplied in value bands spaced twenty per cent apart, too small a difference for our logarithmic ears to discern. If you can't get quite the right value, the next one in the series should do fine.

Metal oxide resistors are almost as cheap as older carbon film types, closer-matched and contribute less background noise. Exact precision is not necessary - within ten or twenty per cent, the circuit will work fine, as long as the resistance values are in the right band of a few kiloOhms. Power ratings, size and orientation are irrelevant. The currents are tiny, and resistors work identically either way round.

The ratio of the value of the resistor connected to the output to that across the input determines the proportion of signal that gets through. That proportion is the total of both resistances, divided by the value of the input one. Amigas nominally output 775 mV, so if your amplifier expects 250 mV, the ratio is roughly 3 to 1; with 1K across the inputs you should use 1.8 or 2.2K series resistors; 2K falls between the standard steps.

If the CD delivers 1.5V AC, use twice the value of series resistor from the CD output as from the Amiga output, to match the signals at the line input. Staying with 1K across the inputs, if the amplifier expects 300 mV you might use 3.9K in series with the CD outputs, and 1.5K from the Amiga's, delivering up to 1.5/4.9 (=306) and 0.77/2.5 (=308) milliVolts respectively.

It's OK to connect one output to several inputs, as long as you don't swap the signal and earth connections over, leading to a loop. Mains hum may be minimised by disconnecting the earth or shield at one end of audio cables, relying on the common power supply to complete the circuit. NEVER disconnect your computer's mains earth!

Audio outputs should only pass AC; if you measure a DC voltage offset in the absence of signal, ypu might need to block this with a capacitor. Add around 22 microFarads of low-voltage Electrolytic or ideally Tantalyum capacitor in each signal lead, with the positive end pointing towards the errant equipment.

Speakers and Power

Specially-made "computer speakers" quality varies almost independent of specification or price. Try to hear them, loud and soft, before choosing. A 6 pair includes a mains power supply but subjects you to "cheap transistor radio" distortion through two three inch speakers. These claimed a rating of 120 watts, followed by the giveaway jargon "PMPO" (Peak Music Power Output). These measure instantaneous peaks and creative accounting, rather than the average power of the sound wave.

PMPO numbers are typically ten to twenty times the sustained average. RMS (Root Mean Square) power values are comparable, but indicate the sustained "heat" the amplifier can deliver - not the mechanical work done by the speakers, let alone their loudness, which depends on the electrical and mechanical efficiency of the drivers and cabinet.

Most computer speakers have 3.5 mm stereo jack leads, compatible with Walkman headphone sockets and cheap PC sound cards. Amiga-friendly twin phono to jack leads are cheap and widely available, sometimes in the same package.

You'll need speakers with an integral amplifier, so look for a battery or preferably built-in mains power supply. You could borrow DC power from the Amigas +5V or +12V supplies, but the bricks supplied with cheaper Amigas have little power to spare. Cheap PC adapters can leach power from ISA slots in Zorro Amigas; most Amiga back panel connectors offer limited DC power.

Software Mix

Once you've combined the signals, you'll want to adjust their relative volumes. Amiga audio applications often have on-screen sliders to do this, but what about CD drives? The front-panel knobs only affects headphones, not the line-level signals on the back panel, and seldom let you tweak the balance for when you're sitting nearer one speaker than the other.

Software can send messages over the SCSI or ATAPI bus, which fade CD audio up or down. AFCD42 programs can mix and balance CD audio as easily as Amiga sounds. If you're "really" keen, use external electronic faders, under RS232 serial control. Relevant instructions and software are on Aminet and the AFCD.

Some programs include CD faders. Oliver Kastl's PlayCD, shipped with IDEfix, AlfaData and Buddha expansions, has a mono slider. Pascal Rullier's freely-distributable SCDplayer 1.2 has volume and balance controls. The shareware Jukebox elegantly combines interacting master, left, right and balance sliders with AREXX.



PlayCD has volume, but Jukebox offers a lot more control

Control GUIs can use commands written for SCSI drives or Commodore's cd.device. ATAPI IDE drives support SCSI commands so "SCSIutil" suits them too. The -v option adjusts four volume controls (forQuadraphonics?!) between 0 to 255.

Commands for cd.device suit CD32 and CDTV, and standard drives via CD emulation, optional with HiSoft's Squirrel and CacheCDFS. They're based on Commodore software standards, working at a higher level than SCSI or ATAPI. The CDToolbox CDVolume command sets replay volumes between 0 and 32767, though practical control is coarser than this range implies.

Internal Mixing

Those who hate DIY might be able to mix CD and Amiga audio by redeploying hardware they already own. The A3000T and A4000 include an obscure connector which inserts stereo signals into the audio chain, after the filter and before the final output buffers. This is ideal for CD mixing, perhaps tweaking the CD output balance with software. This three pin motherboard connector lurks close to the phono output sockets; consult figure D-1 in the A4000 manual, or look for JCDINP between the video slot and Paula chip inside an A3000T.

The sneakier way is rather more demanding of your system, but can work well on expanded Amigas. You can read audio from the CD drive in digital form, typically over SCSI, and mix them with program-controlled samples, on the fly. Digital audio transfers are not a mandatory part of the SCSI spec, or in cd.device. experiment with drives and drivers to find a good match. Some drives play audio more reliably when slowed down to single speed. It's more important to pick up every block first time than to boost the average speed by spinning the disk faster, then needing to re-read blocks, causing audible stutter.

This is a CPU-intensive process, especially as you'll probably be using AHI for sample output, but it requires no external mixer - the mixing is done digitally, and the combined sound comes out of a single stereo line output, from a 16 bit sound card, or Paula at a pinch.

Tying Up

Prelude expansion hardware for Zorro or A1200 includes a software-controlled analogue mixer. This combines signals from the CD drive, Paula and the 16 bit sound card into a single stereo output, with independent volume controls. This is the ideal option once you've got it all wired up; the cabling gets intricate, especially as Prelude also offers low-level and line inputs.



ACT's graphic front end for Prelude's mixer



The GadTools front end for Prelude's multichannel stereo mixer



The MUI version of the mixer shows levels numerically

Delfina boasts CD audio inputs as well as line sockets. Simpler sound cards might need to sample the Amiga output and mix it with AHI toget everything on one pair of plugs without extra hardware. DIY adapters are a more flexible and CPU-friendly solution.

An outboard mixer is ideal, space permitting. I use a Soundlab three way stereo switch and an Amdek MXK600 video sound mixer to master audio for AFCD. There's no limit to the audio potential of an expanded Amiga, but if you're preoccupied with listening, rather than original recording, the little adapters described here are all you need to combine the benefits of data disks, native, CD and sound card audio.

Contacts

Resistors, potentiometers and screened audio cable should cost just a few pence from Tandy and local electronics suppliers, or these UK based Internet-aware firms:

www.cpc.co.uk - Spares, CD Audio cables, switch boxes etc
www.farnell.co.uk - Components and Amiga-friendly webmastry
www.maplin.co.uk - Retail and mail order electronics catalogue

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