Sauce For The Goose – A case for digital signal paths in car

Sauce For The Goose – A case for digital signal paths in car audio systems


Welcome to those who followed the link from our August Driving Sounds Club newsletter. I have reused the image you initially saw to assure you that you are in the right place. For those who have found a different way to this blog, the image will mean little unless you go to and become Driving Sounds Club members but read on if you are looking for information on audio signal paths particularly in a car!

The inspiration for the clumsy “Sauce” vs “Source” pun is the thought that one’s idea of a perfect sauce to accompany this majestic table fowl can differ hugely from another’s. Some like it sweet, whereas I prefer rich and savoury with the fruit (grape) accompaniment in a chilled glass on the side! Similarly, some like to purchase a physical media from which to listen to their music like a CD for instance, while others solely listen to one or a variety of downloaded file types delivered by portable devices in a number of different ways.

I prefer CD. I can remember where all the tracks are on a CD and can easily navigate my way through them without suddenly finding myself on a random musical excursion caused by a clumsy press of the wrong button on my impossible-to-read-whilst-driving mobile phone screen. I have to say that my preference is no longer driven by a distrust of the downloaded music files available. Having listened to Apple lossless files vs WAV files vs Flac files I am pleased to report they all deliver source information of adequate quality to get my feet tapping to a greater or lesser degree. The biggest discoveries we are making are related more to the way that source information is delivered through the various elements of your system to the speakers.

4-series dash

I recently sat in a BMW 4-series with a very good system installed in it with the installer and a highly experienced car audiologist. We had some time for a change and were able to try the same tracks from two different sources those being Apple Music and CD. We were left concluding that CD was outperforming the same track played from an iPhone by quite a large margin. This was until we reviewed how we were getting music from the iPhone. Initially, we used Bluetooth. We were quite surprised as results “on the bench” have never shown such a marked difference between airborne transmission and directly connected sources. We noticed a USB port on the car and thought it worth plugging the phone directly into this by way of comparison. This made a very positive improvement reinforcing my strictly non-scientific assurance that Bluetooth would always be at best a little hit and miss quality-wise and probably varies in quality depending upon the robustness of the signal, levels of interference, and proximity to the receiver.  So, what else was different? Don’t forget, we were simply using our ears here so, no science involved. Three of us listened to both the CD and Bluetooth versions individually and all concluded that the Bluetooth version lacked dynamics. it sounded a little squashed, particularly in the bass registers. Image width, depth, and height were also affected. We then listened to the difference between Bluetooth and USB and again all three of us independently noticed a distinct difference in dynamics with the USB connected phone seeming to do a much better job. Finally, we compared USB connected phone with CD and this time the CD won hands down although the difference was not as marked as that between Bluetooth and CD.

On investigating the installation more closely we discovered that the CD source was being picked up directly from the MOST bus in digital form.

Car Audio is well and truly embracing the digital age in which we live. The adoption by motor manufacturers of Media Oriented System Transport  (MOST) is testament to that. It enables audio signals to be transported in the digital domain to other devices. However, in this particular model, the ability to do this is only switched on if you buy the very expensive Harman Kardon upgrade from your BMW dealer. The installer of the system is ultra-experienced engineer from FOUR MASTER John Kleis, Simon Longmuir. Simon explained that he had found an independent car repairer who has managed to gain access to the software switch required to enable MOST. This means that the pure digital source can be picked up, processed, and amplified in the digital domain. No loss, and very little chance of interference or degradation by other on-board systems. This is a truly excellent discovery as it enables installers to pick up a genuine first-hand digital source without customers having to spend a fortune on an upgrade, the most significant element of which is a swiftly executed switch in the head unit’s software. The results are startling and have gone a long way to explaining earlier assurances that BMW head units are a pile of embarrassing, odorous disappointment! In actual fact, the head unit does a great job of extracting digits from a disc and passing them on. It is what happens next in the standard system that lets everything else down.

For those sceptical of this or their own ability to hear the difference, I would urge you to visit John Kleis ( and listen to this excellent installation as the vehicle is on their demo fleet. Other FOUR MASTERS ( may also have now managed this so it is well worth checking with your local one first!

For reference the full system comprises:

Audison DMI bit

Audison Prima 8.9bit

Audison Prima 1D

Rainbow IL 3-way speakers

Rainbow Germanium 10″ subwoofer


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