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Room correction - why the needless analog to digital conversion?

by Nyal Mellor July 24, 2010

Last week I talked about the importance of letting any end user choose the frequency range to which room correction is applied and allowing them to specify a target frequency response. In this weeks article I cover the last two functional criteria for a room correction product. Whilst these are less important than the first two, they are still important, and just make good sense! This week I cover the importance of not performing needless analog to digital conversions and providing measurement and filter generation capabilities.

Must not perform an un-needed analog to digital or digital to analog conversion

Many DSP room correction devices (even expensive ones!) only provide an analog input (e.g. Audyssey, Velodyne SMS-1) despite all the processing being performed in the digital domain. When a user is using a digital front end as a source such as a music server or CD transport it makes no sense to force additional conversions into the signal path. All DSP based room correction devices should therefore include a digital input capability. I’m not sure why more companies don’t provide a digital input, maybe it is the cost of engineering a low jitter clock recovery scheme relative to just adding a analog to digital converter chip. And especially when you think that if a product does not include a digital input then there are TWO conversions that take place - a conversion from analog to digital on the input and another conversion from digital to analog on the output! TacT and Meridian are some of the companies that have a digital input capability. ADDED 8/5/2010 - Since all the home theater processors that I know of do not have digital outs, there is no requirement for an outboard room correction device to have a digital input. Of course most modern ones include digital room correction algorithms so the need for an outboard box is reduced. Thanks to Alan Langford from DEQX for pointing this out!

Must provide automatic measurement and correction filter generation capabilities

Any product hoping to have good penetration into the marketplace and consistent results when implemented needs to build in some form of measurement and correction filter generation capability. Even products designed to be installed by a dealer (e.g. Rives PARC) can be inconsistent when the vagaries of dealer expertise and experience is taken into account. By not providing any measurement functionality there exists the need for acquiring a separate measurement rig, typically comprising acoustical measurement software (either provided by the vendor or a general 3rd party package such as ARTA), a calibrated measurement microphone and associated cabling. It is not just the cost of acquisition that is a burden, there is also the learning curve associated with its use. Some companies such as Audyssey have integrated a lot of ‘failsafes’ into their software such as level checking and guided measurement processes. I applaud these companies for their efforts. Others such as those based on parametric equalizers such as the Z-Systems RDP-1 or the EQ within Sonic Studio’s Amarra rely totally on the end user being able to set measurement levels properly to get a good signal to noise ratio and know when a measurement should be repeated. There is however, a very fine balance that must be struck between ease of use and richness of functionality. In my opinion many products overstep the line, focusing too much on ease of use and compromising on functionality, often not meeting the first two functional criteria. On the flipside it is also easy to go too far in the other direction and create a product that can only be understood and implemented by a very patient or knowledgeable user.

Well, that's covered my functional criteria for a room correction product. Next we will survey the market to see what room correction products are available and how they meet these basic functional criteria.

Do you think I've covered all the functionality that a room correction product must provide? Let me know via through the comments!

Nyal Mellor
Nyal Mellor


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