® Technical Paper Number 103 . . . Level Matching for Autosound . . . ?or Why is my system so noisy? by Richard Chinn All Rights Reserved. Copyright 1986. Rick Chinn is a working sound engineer who has been involved in all facets of recording, sound reinforcement and acoustical design for the past fifteen years. He has written articles for Modern Recording and Music, Music and Sound Output and A/V Video and has worked for TAPCO and Kaye-Smith studios.
Contents Page Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 What is noise? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Thermal Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Induced Noise . . . . . . . . . . . . . . . . . . . . . . . .
® . . . Level Matching for Autosound . . . ?or Why is my system so noisy? Introduction With the advent of really high-performance automotive sound systems, the old problem of noise (in its various forms) has once more raised its ugly head. Even if the manufacturer has done their engineering homework, it’s still really easy to forget some detail during installation and make even the quietest components seem noisy. Consider: • A head unit with an integral power amplifier.
® What is noise? In any audio system, there are several potential noise sources. • thermal noise • induced noise • noise caused by a ground loop Thermal Noise Thermal noise sounds like the hiss that you hear between stations on your FM radio. Thermal noise occurs because mankind chooses to live at 300 degrees above absolute zero. Thermal noise occurs because of the random movement of electrons caused by thermal agitation. At absolute zero, thermal agitation ceases to exist.
® What is Noise? Ground Loops Ground loops are insidious. They are caused by the non-zero resistance of the wire used to interconnect the equipment. Typically, ground loops are created by a piece of equipment having multiple connections into the grounding system. In autosound, this problem is exacerbated by the rather callous belief by auto makers and certain other folks that the metal frame of the vehicle makes a good ground.
® The Meaning of Signal-to-Noise Ratio Signal-to-noise ratio is a common term used on spec sheets. As common as it is, many people (both learned and not) misunderstand or misuse it. Signal-to-noise ratio is the relationship between a device’s normal operating level, the noise floor, and peak clipping. The noise floor, or the Ultimate noise All electronic equipment operating above absolute zero generates noise.
® Meaning to Signal-to-Noise Ratio Figure 3. The relationship between operating level, clipping and the noise floor. As you can see, the operating level chosen for any given device has a pretty direct effect on the working signal-to-noise ratio. Pick a level that is too high and you get premature peak clipping (but spectacular noise performance). Pick a level that is too low and you have lots of headroom, but a poor working signal-to-noise ratio.
® Gain, Level, Sensitivity, and Power Relationships Another source of error is the interrelationship between gain, signal level, sensitivity, and output power. First some definitions: Gain Generally this is shorthand for voltage gain. A measure of the amplification factor of an amplifier, or the output voltage divided by the input voltage. Thus, 10 volts out for 1 volt in is a gain of 10. Sometimes gain figures are converted into decibels (dB).
® Gain, Level, Sensitivity, and Power Relationships Another reason is to maximize the amount of useable range that the volume control has, which is especially useful with units that have detented (click-stepped) volume controls. The volume control taper (rate-of-change vs. amount of rotation) at the low end of the control is necessarily coarse. After all, most people like to be able to shut the sound off once in a while.
® Minimizing Noise in Autosound Systems Now that we’ve established some of the causes of noise in any audio system, let’s apply them to a typical autosound system. First let’s make some assumptions: 1. The manufacturer did their design homework and the unit represents the best that they could 1. do, considering their design constraints. 2. The unit meets its published specs.
® Minimizing Noise in Autosound Systems Figure 4. Bypassing the alternator output. Grounding practices CAUTION Ground Isn’t Ground! Remember those words. Any time that you think that grounding is a trivial matter recite those three words. The basic reason that we ground anything at all is to supply a negative return for the electric current that operates a device. If you’re an auto maker, basically anything that is metal and somehow associated with the vehicle body is fair game.
® Minimizing Noise in Autosound Systems 2. If you mount everything on a board, connect the chassis of each component to a good, solid electrical ground. The vehicle body is a good choice here because what you want is a good RF (radio frequency) ground. The wire running to the negative side of the electrical system has inductance which raises it’s impedance at high frequencies. In effect, at radio frequencies, the impedance could be high enough that the wire would look like an open circuit.
® Minimizing Noise in Autosound Systems Figure 5. RF bypassing the power supply lines. Level matching Last but not lease is level matching. I’m presenting this last because all the other aspects of the system must be right first. Level matching helps minimize hiss: the residual noise output of the head unit. Level matching can help make other noise sources (alternator noise, etc.) less evident by making the useful signal much louder than the noise.
® Minimizing Noise in Autosound Systems 3. Now adjust the sensitivity control on the power amplifier until you hear it clipping, or the system is too loud. Remember: you want as much signal going through the cable between the head unit and the power amplifier as possible. Interfacing with internal power amplifiers Sometimes you have no choice but to use an existing powered radio/head unit as the front end for a sound system.
® Minimizing Noise in Autosound Systems Connecting a normal(one side of the output is actually connected to ground somewhere in the system) radio to an equalizer or power amplifier is easy. 1. Somewhere near the radio, cut the wires short and splice them into a piece of shielded cable. Run the cable to the equalizer or amplifier and terminate it in a compatible connector.
® Conclusion We’ve covered a lot of ground in a short time. Some of the concepts presented may not be crystal clear the first time through. Take the time to read them through and attempt to understand them. Don’t let the numbers scare you; you can skip them until you get deeper into the subject. The subject of noise is not a simple one. There are many different noise sources, and perhaps twice as many cures. You can take a shotgun approach to the problem and perhaps be successful 50% of the time.
® Appendix A Most noise problems with the AudioControl EQX Equalizer/Crossover and EQL Equalizer can be traced to improper level setting. The optimum signal level through the EQX or EQL is 1 volt. This is considerably higher than the output of most head units. Both units have adjustable gain input stages which boosts the input signal up to the 1 volt level. Since both units have bi-polar power supplies, they can operate with 1 volt signals without sacrificing headroom.
® Appendix B Final Setup for the EQX The initial part of the setup for the EQX is covered in the previous section. 1. Set the HIGH FREQUENCY and LOW FREQUENCY output controls of the EQX to 12 o’clock. Advance the head unit’s volume control toward 75%. If the amplifiers clip, or if the system gets too loud, you are done.
® Appendix C Procedure for Powered Radios Using the EQX/EQL with head units with internal power amplifiers isn’t all that difficult. 1. Determine whether the output is floating or grounded. Use an ohmmeter on both output wires (hot and ground) from each channel to the negative power connection. If the output is grounded, you will read a very low resistance (less than 1 ohm) between one of the output wires and the power supply negative lead. 2.
® ® 22410 70th Avenue West, Mountlake Terrace, WA 98043 (425) 775-8461
