Car Audio

In the 80s and 90s, it seemed like car audio installers were often having to resolve noise issues after installing a system that included an amplifier or signal processor. In many cases, the problem was called a ground loop. However, in some cases, the noise was present because one of the signal cables passed by a source of noise. The only solution was to reroute the wire. No matter what the problem was, or what cure was applied to resolve the problem, ultimately it wasted time and frustrated installers.

Noise problems are not as common these days. They do crop up from time to time, but certainly not with the same frequency as before. One simple technology that has helped to reduce noise issues is the inclusion of differential inputs of modern amplifiers and processors. In this article, we will explain what differential inputs are, how they work, and what benefits they offer.

Amplifier Input Circuitry – Single Ended Inputs

The RCA input connections on your amplifier have a relatively easy life. They do not need to pass any significant amount of current. They do not get connected and disconnect very often. They just sit there and do their thing.

Behind the connections, where the circuitry of the amplifier starts to do its job, there are two circuit designs commonly used. The aforementioned differential input, and single ended inputs.

In a single ended input design, the shield of the RCA cable is connected to ground through a resistor. This resistor is often around 1000 Ohms. The center pin of the RCA cable, which contains our audio signal, is connected to the preamp stage of the amplifier. The preamp stage includes the sensitivity control, crossovers and so forth.

The image below shows an RCA with a sine wave and noise on both the shield and the signal conductor.

When we feed the signal shown above into an amplifier with single-ended inputs, the noise on the ground wire is ignored, and the sine wave and noise on the signal wire are amplified. The unfortunate and unwanted result is below:

Amplifier Input Circuitry – Differential Inputs

In a device with a differential input design, the shield of the RCA cable is completely isolated from the chassis. Before the audio signal passes to the preamp stage of the amplifier, it goes through an op-amp. An op-amp (or operational amplifier) is a small IC that has two inputs and a single output. One input is labeled as +, and the other as -. When we put a signal on the + input, it is passed to the output of the device with little to no change. When we put a signal on the – input, the polarity of the signal is inverted and sent to the output. In a differential input configuration, the – input of the op-amp is also connected to the shield of the RCA.

If we have an audio signal on the + input, and nothing on the – connection, the audio signal is passed through to the output of the op-amp, then on to the pre-amp stage of our amplifier.

The image below shows an RCA with noise on the shield and the signal conductors.

If we have noise induced on the shield and signal conductor of the RCA, then the noise signal on the shield is inverted and passed to the output. The inverted noise signal adds to the non-inverted signal and cancels out.

We call the system a differential input because it is looking for the difference between the shield and the signal conductors. More accurately, it subtracts any signal that is common to both conductors from the output signal.

How does Noise get into the System?

When our interconnect cables run through the vehicle, they come close to all sorts of electrical noise devices. High-current wiring, computers, sensors, electric motors and even the alternator can produce radiated noise in the form of a pulsing magnetic field. When a conductor passes through this field, a current will be imposed on the wire.

When this imposed current (or signal) reaches our amplifier, it gets amplified. If the noise is a high-frequency buzz, then that buzz is amplified, and we hear it through our speakers. Yuck!

Another source of noise is a ground loop. In a ground loop, current is flowing on the shield of our interconnect to equalize the ground potential between two devices.

Let’s look at an example. Perhaps someone has installed a radio in the dash of a truck and grounded that radio to the dash support. In some cases, that dash support is isolated from the truck chassis with big rubber bushings. When we connect the RCA cables from our radio to an amplifier in the trunk that has single-ended inputs, the radio seeks an improved ground location through the RCA shield. The resistor in the amp prevents massive amounts of current from flowing, but there is often enough to induce noise in the signal conductor.

A quick test for a ground loop is to pull the RCA out of the amp until only the center pin is touching. If the noise goes away, you have a ground loop. A remedy is to install a ground loop isolator. A ground loop isolator is a device with a transformer in it. The audio signal is transferred through the magnetic interaction of the transformer windings. There is no direct electrical connection between the input and output cables. With no electrical connection present, current cannot flow. The drawback of a ground loop isolator is that it can detrimentally affect the frequency response of the signal passing through it.

Do the Interconnects Matter?

For differential inputs to work, we need the noise signal to be equal on both the signal and shield conductors of our RCA cables. In a coaxial cable design, the shield can block noise from getting to the center conductor. If you use coaxial interconnects with differential inputs, the noise on both conductors is not equal and can induce noise into the signal path.

The easiest way to ensure that any induced noise is equal on both conductors is to use a twisted pair interconnect. In a twisted pair configuration, both conductors have the same effective amount of shielding and subsequently, the same amount of noise.

Differential Inputs are Your Friend

The next time you are shopping for an amplifier or signal processor for your car, remember that one of the questions you should be asking is if it has differential inputs. While you can certainly build a great sounding audio system around an amp with single-ended inputs, there is no point of taking the risk of having noise or installation headaches. Your local mobile electronics specialist retailer would be happy to assist you in finding an amp or processor with differential inputs.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.

Awhile ago, a client asked us what you get when you buy a “better” headunit. The usual answer is that you get more features and improved functionality. Those of us with a drive for the ultimate in sound quality and realism from our mobile electronics systems choose products in hopes of them receiving improved sound quality. Beyond frequency response and noise, what else makes one radio sound better than another?

We invited two head unit specimens to the Best Car Audio test bench for a little head-to-head battle. The first subject is a modern multimedia station that is equipped with navigation, smartphone integration and the general Bluetooth goodness. The challenger is a veteran, but premium, CD receiver. It has never heard of MP3 files, doesn’t understand the concept of satellite radio and thinks Bluetooth is the result of eating blueberry pie. However, in its day, it was one of the best. We do not need to mention names here, but we will call this the bench battle of features versus performance.

The Equalizer – Premium Test Equipment

Measuring the frequency response of a car audio product is relatively easy if you can feed a known signal into the device. When it comes to measuring a signal source, that is responsible for generating the signal, so all we can do is measure the output. We have a high-end digital interface on our bench. It offers a flat frequency response from 5 Hz to 92 kHz with a tolerance of 1 dB. The signal to noise ratio is an amazing 116 dBA and distortion is specified at less than 0.00032%. These specifications exceed those of both source units we are going to test.

Frequency Response Measurement

Testing the frequency response of a source unit requires some trickery. We have developed a reliable method that has proven itself time after time. The image below shows the frequency response of the reference signal. It has a slight incline in the high-frequency region, but everything is within a tolerance of about 1 dB. We converted this 192 kHz, 24-bit test track down to the CD standard of 44.1 kHz and 16 bit. Response to 22.05 kHz remains ruler-flat.

Frequency Response Results

Before we get into the results, we want to explain how to use the measurement graphs. The test track uses random noise as part of the test procedure. We process that after the test is complete. What you want to observe is the trend of the charts. A small peak or valley is not an anomaly in this scenario. Average the curve in your mind to see the overall trend.

The CD receiver: We played our test track from a standard CD audio file to make sure both source units were given the same information. The frequency response of the unit showed a typical response. The high-frequency filter kicks in around 17 kHz, which is normal for consumer products.

The DVD receiver: We played the same CD in the DVD receiver to see how it responded. The manufacturer of the DVD receiver has included a 2 dB boost on the top end that starts at 7 kHz and peaks at just over 2 dB at 15 kHz. The high-frequency filter response is similar to that of the CD player.

The high-frequency boost is not a big deal in terms of how a system sounds. Most of us have some high-frequency attenuation in our hearing, so this helps put some of the sizzle and air back into our music. It would be worth checking whether the source unit output clips when a 0 dB 15 kHz tone is played. This article is not a product review, so we will save that for someone else to tackle.

Bring the Noise

Our next challenge for the new versus old shootout is a little more technical: We wanted to see how each unit performed regarding background noise. This test is often reserved for lab environments, but can quantify the effort put into the component selection and system design.

For this test, we used a 1 kHz test tone recorded at -90 dB relative to full scale. Because the two source units have different pre-amp capabilities, we adjusted them so the output of the 1 kHz tone was equal in amplitude. This would best depict the noise imposed on the signal.

The CD receiver: We can see that the background noise relative to the signal is very quiet. There is a little bump at 60 Hz that was created by the power supply on our test bench. Otherwise, the test was impressive.

The DVD receiver: The background noise relative to the 1 kHz is 10 to 15 dB louder than that of the CD receiver at higher frequencies. There are also some spurious harmonic distortions in the output signal, mostly above 1 kHz. The large bump in noise in the low-frequency region could be caused by our 60 Hz 120 volt power supply causing some harmonics. The bandwidth is really wide, so it is hard to determine for sure.

A Distorted Perspective

We decided to repeat the test with a full-amplitude 1 kHz sine wave to see what harmonic content would be created at higher internal levels. The limits of the FFT analysis in our software starts to show up here. The flat horizontal line on the left of the chart and the angled line on the right are due to the analysis software and don’t represent noise.

The CD receiver: We noted two small harmonics at 14,750 and 165,000 Hz, with a slight harmonic at 12,000 Hz. Otherwise, the signal was very pure.

The DVD receiver: Harmonic distortion was clearly present at 1 kHz intervals starting at 2 kHz. It is worth noting that the first resonance is 45 dB quieter than the reference signal. If you were just playing the test tone, you might be able to hear it, but only just barely.

The Grand Finale – Intermodulation Distortion

Testing for intermodulation distortion is, well, mean. Out intermodulation distortion test is comprised of a CD test tone with 19 and 20 kHz sine waves played simultaneously. The spectral response of the test track can be seen below.

The CD receiver: When you are looking at an intermodulation comparison test, you are looking for frequency content that wasn’t in the original file. In the case of the CD receiver, we can see a very small bump at 1 kHz. This is significant because it is the difference between 19 kHz and 20 kHz. This would be considered an excellent result. A few other spikes show up at 3,500, 4,500 and 9,500 Hz, but they are still quite low – peaking at -93 dB relative to the test signal level of –15 dB.

The DVD receiver: We cannot really explain what happened here. There is 1 kHz content only 20 dB down from the 19k Hz and 20 kHz tones. Then harmonics upon harmonics of this up to 21 kHz. This test shows why some source units sound accurate and pure, while others do not.

Are Headunit Features Worth the Trade-off?

A few months ago, we published an article about harmonic distortion. That served to establish the basic understanding of how distortion creates content that wasn’t present in an original audio file. Our intent in this comparison is not to put down the modern DVD receiver, but to show what happens when manufacturers forego the bells and whistles and simply focus on all-out performance.

The same tests that apply to these source units are also common in amplifiers and speakers. We will subject an amplifier to the same mean and nasty tests in the coming months.

Don’t ever let price, perception or age dictate how you think a product sounds. Work with your mobile electronics specialist retailer to listen and compare for yourself. You will be amazed at what you hear.

This article is written and produced by the team at www.BestCarAudio.com. Reproduction or use of any kind is prohibited without the express written permission of 1sixty8 media.