Optimising digital signal levels - v4 with Reaper

[organsRgreat]

I’m running version 4 in conjunction with Reaper (version 4.611 from 2014), which was necessary in the old days in order to add reverb. To provide multiple channels each HW output feeds a separate channel in Reaper; then a mixdown feeds into Liquidsonics reverb, which has its own speaker channel.

Basically the setup works well, but I’m hearing distortion on big chords, so there must be a digital overload somewhere in the audio chain. In the days of analogue the idea was to feed the audio signal at maximum available voltage into the mixer (to get the best signal to noise ratio), then adjust gain controls so that the individual channels and the master bus in the mixer weren’t overloaded.

Does the same apply when most of the system is digital? Should I keep the Volume and Trim controls in HW at their maximum settings, then use Reaper’s volume controls to reduce the level until distortion disappears; or does it not make any difference? Are the volume controls on Reaper tracks equivalent to gain controls on an analogue mixer?

I know that with digital audio everything has to be kept below 0dB, but apart from that I’m confused because I don’t fully understand how analogue concepts apply to digital.

[mnailor]

I can't answer for Reaper, but if your Hauptwerk Audio level meter (on the same control panels as Trim and Volume) ever shows red at the highest playing level, your digital output from Hauptwerk may be clipped or distorted. You need to reduce that meter to green and occasionally yellow for all your samplesets played hard at full organ.

Then you can start adjusting levels/gains/sensitivities in Reaper, audio interface, and amplifiers.

Also, you need to monitor HW's CPU meter (and Reaper's if it has one), and also run LatencyMon to look for high DPC and ISR latencies, to make sure your CPU isn't overloaded, causing buffer underruns anywhere in the audio chain. That is another source of distortion which has nothing to do with volume, but is just as likely.

[organsRgreat]

Thanks Mark that’s brilliant – exactly what I need to know, and so clearly explained. I’d forgotten about Hauptwerk’s audio level meter, and it hadn’t even occurred to me that the PC’s CPU might be overtaxed. This is quite a powerful machine but obviously it has its limits. I’ll follow your suggestions and see how I get on.

[organsRgreat]

I’ve checked quite a lot of things, and just today got two interesting results. There’s a problem when I play 4 note chords an octave apart, very fast. They start out OK, but after about four pairs of alternations the sound starts to break up, and after another four there’s silence. This happens with a small registration – 8 and 4ft flutes from a unit rank. So I tried using Hauptwerk 4’s MIDI recorder, and playback from that is perfect!

I’m now running LatencyMon. After five minutes it said “Conclusion: Your system appears to be suitable for handling real-time audio and other tasks without dropouts”. But now that it’s been running for over an hour it says “Your system seems to be having difficulty handling real-time audio and other tasks. You may experience dropouts, clicks or pops due to buffer underruns. One or more DPC routines that belong to a driver running in your system appear to be executing for too long. One problem may be related to power management. Disable CPU throttling settings in Control Panel and BIOS setup. Check for BIOS updates”.

I’m using a 4 manual Hoffrichter console that does all MIDI merging internally, so there’s just one MID Out port (and one MIDI In which is useful for lighting up pistons). The MIDI Out from the console goes to the MIDI In of a Motu 828 hybrid Mark 3, connected over firewire. In fact I have two identical 828s, with one linked to the other over firewire, so that my PC sees them as one large unit.

My PC processor is an “Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz, 3696 MHz, 6 Core(s), 12 Logical Processor(s)” with 32GB RAM, running Windows 10. I’m happy to supply more information if necessary, but hope the above may be enough to suggest a way forward. Do I need do adjust buffer settings somewhere?

[mnailor]

When this sound breakup happens, what organ are you playing, and what perspectives (Front, Rear...) are loaded if it's a surround sampleset?

What is your polyphony limit, sample rate, and sample buffer size? Do those match the sample rate and buffer size on both 828s?

What are the highwater marks on the Hauptwerk CPU meter, polyphony meter, and audio meter immediately after this glitch? For example, high green, low yellow, high red...

How much RAM does the Hauptwerk RAM meter show is free?

In the LatencyMon reports (driver tab maybe, not in front of me now), what drivers are blamed for the highest ISR and DPC times?

Most importantly, have you done all the work in the Performance Tuning chapter of the Hauptwerk Installation and User Guide to your PC? This kind of problem is why it's there. It can take two or three days to do all the steps systematically. If that doesn't find the problem, download the Glitch Free PDF from Cantabile's web site and study it for additional measures to try.

For setting expectations, the i7-8700K is a medium capacity CPU in modern terms, so large samplesets are possibly beyond it. But audio glitches when playing only a few stops with 8 notes fast points to a lack of performance tuning on Windows.