- miniDSP encased, thus more like a modification than own project
- Integration with Room EQ Wizard (REW) or manual adjustment of numerous parametric filters
- Analog RCA and digital SPDIF toslink/RCA connections
- Input selector and (digital) volume control for preamplifier use
The name miniDSP refers to a company in Hong Kong which is developing and manufacturing audio DSP-solutions for hobbyists and OEM-manufacturers, but is also a general name for their audio DSP products. The project described here is not my own design but uses the basic miniDSP kit which I have only encased in an aluminium enclosure along with few modifications. Thus, this is more a description of the capabilities of miniDSP. This project of mine is a step towards the DACpreAMP project which will be connected to this box, forming a complete audio system.
miniDSP is 2-input/4-output board with on-chip AD- and DA-converters, thus containing analog RCA connectors. Separate miniDIGI cards are sold to offer digital connections and such a card is also used in this device. So, the specifications are
- Input, 2 channels: RCA analog, 2x SPDIF toslink, 2x SPDIF coax (these 5 inputs are selectable by rotary switch)
- Output, 4 channels: RCA analog, SPDIF toslink and SPDIF coax (only one of the two output pairs at the moment, both SPDIF outputs transmit the same signal)
- Versatile crossover filters (for subwoofer(s), for example) and other digital filters to offer parametric equalization, also possibility to type custom filter coefficients
Products of miniDSP are mostly sold as bare PCBs with required level of wiring diagrams and datasheets. Full schematics are not provided. However, the user of miniDSP needs to have rather good electronics knowledge if implementing them really part of other designs, using I2S-buses. On the other hand, just plugging or soldering cables does not require too much understanding of electronics, just digital audio and audio in general. For developers there is also a support forum on miniDSP site where answers for the questions outside datasheets can be found. However, datasheets provide almost all the information when read and understood carefully. For easiest possible deployment, this model I have used is also available in plastic enclosure, unfortunately without digital connections.
DSP in Audio
DSP hardware by miniDSP is sold separately from software called plugins. Plugin determines the operation of the DSP. The boards can be used for active crossovers for active DIY speaker designs but in this case it is used as a parametric equalizer to perform digital room correction, correcting some flaws caused by room acoustics, mostly in low frequency region. The best plugin for that is 2-way advanced 2.1 (PDF datasheet).
Internally miniDSP has programmable digital biquad filters. These 2nd order recursive filters can be cascaded, thus offering possiblities for very steep filters when needed. Plugins also support custom coefficient entering, if easier to understand Parametric EQ (PEQ) filters are not enough. For the easiest use there is a support for Room Eq Wizard (REW) where the measured room response can be used directly to calculate the filters for room correction. As the name of the plugin implies it can be used in 2.1-configuration with crossover for subwoofer and also separate PEQs for main channels and subwoofer. Of course, it can be also used in 2.2-configuration where subwoofer channels are not summed but kept separately with individual filter configurations. There are also separate gain, polarity and delay settings for each channel.
Plugins consist of Windows software used to define the filter configuration and program DSP-chip. Experimenting is easy since filter tuning can be done in real-time.
..I don’t have a measurement microphone to use with REW and so far I have used micw with my phone to measure frequency response and tune filters, along with listening tests.
Room Acoustics Measurements
Measurement microphone and appropriate software are needed for accurate measurements of room acoustics, which are the basis for tuning the DSP-filters. Sine signals and ears are one possibility but for more accurate analysis of problematic frequencies, measurement is essential. After correcting the worst flaws, or at least doing as much as one can, the sound can be fine tuned based on personal preferences.
Common hobbyist measurement system is cheap measurement microphone such as Behringer ECM8000 with a microphone preamplifier including phantom power supply, and Room EQ Wizard software. Probably the smallest solution is MicW i436 calibrated measurement microphone intended to use with Apple iDevices and suitable app such as Analyzer. The app offers tuning the parameters of FFT to affect how the spectrum of the measurement is presented. Results can be exported in PNG or CSV. Some averaging possibilities are offered. The app also supports MicW i436 calibration as in-app purchase, correcting the high frequency roll-off, I assume.
I bought this microphone few months ago, for future use, but have only now used it the first time. To be honest, now I would probably go for the measurement mic by miniDSP. I had iPhone 3 GS what I was planned to use with MicW. I still have it but the phone I actually use now is Asus PadFone 2 (Android). I did the first experiments with MicW using iPhone 3 GS but I cannot recommend that configuration! Maybe with new iPad it is ok but iPhone is way too small for this, and an old iPhone is way too slow although it is supported by Analyzer. Good app could make it insignificant but Analyzer’s data export functions are not the best and it is just very tacky to use. For random use it is ok but otherwise not. For sure iPad improves the user experience significantly, especially if screenshots are also larger.
Very nice feature of Analyzer is the included pink noise, white noise and sine wave generators. Using the Y-cable provided with MicW it is possible to plug both the microphone and speakers to phone/tab.
MicW i436 itself have relatively flat frequency response, at least in lower frequencies which are usually interesting for audio enthusiasts. In high frequencies there are some fluctuations but I assume unit-to-unit variations are not significant. Microphone includes individual sensitivity value for rather accurate sound pressure levels as well, if needed. However, the problem with this mic can be the variety of devices to plug it in. That is why it is recommended for iDevices since their frequency responses and features are known. For Android devices the situation is different. However, I will come back to this topic of room acoustic measurements later when I have found good Android software and reliable workflow for measurements. I also need to study the subject a bit more and experiment to draw conclusions and make actual room corrections. Nice thing is that everything can be done in real-time when experimenting.
There will be some additions to miniDSP before using together with upcoming preAMP and finally DACpreAMP. I will add a simple power supply inside the enclosure which will be fed by a mains power supply from preAMP or DACpreAMP. This will eliminate one mains power cord but mostly the reason is that the power supply of miniDSP board is quite sensitive to power disturbances. When powered from USB, slight jiggle of the USB cable near connector, apparently leading to a very short power shortage, is enough to shut down the DSP followed by a loud thumb from speakers.
Another addition will be a possibility to change the input of miniDSP with upcoming preAMP or DACpreAMP. There will be control signals along the aforementioned power cord to provide the functionality.
Also another SPDIF-transmitter must be added to be able to transmit both output pairs as digital signals, to get 4-channel digital output for 4-channel DAC.