Obsessive compulsive manual DRC in REW/rePhase

[Keith_W]

Any crossover that causes a phase shift will be harder to design. I said "harder", not "impossible". For example, here is a simulation of a minimum-phase 4th order Butterworth XO at 80Hz:

Red = LPF, Green = HPF, Brown = summation. All IIR crossovers, and all passive crossovers with the same 4th order BW will have that 3dB hump at the XO region. And that's the electrical crossover only. Once the driver is involved, there will be additional phase rotation so who knows what the result is going to be. The linear phase version sums perfectly flat. And because you can manipulate phase independently of amplitude, you can also make the summation with the driver flat.

You should understand what you are getting into. In a nutshell:

- Passive crossovers: minimum phase. Wastes amplifier power as heat. If you wish to do more correction, you increase the complexity of the network and waste even more amplifier power and component prices go up. There is no possibility for room correction. And furthermore, every driver is different thanks to manufacturing tolerances. Passive XO's have a very limited ability to deal with that.
- digital IIR crossovers: the digital version of minimum-phase passive crossovers. (Minimum phase means that the amplitude is inextricably tied to phase). It is a huge step up. Steeper slopes and more complex corrections can be achieved without wasting amplifier power. You can do room correction, but your ability to do so is more limited than a lin phase FIR.
- digital linear phase FIR: these do not exist in the natural world. A constant delay is applied to the signal allowing independent manipulation of amplitude and phase.

MiniDSP's have their place. They are inexpensive, do a great job, convenient, robust, and they measure extremely well. It's a great product. But what you can do with them is somewhat limited, and if you like to tinker you will start running into its limitations. The versions with Dirac automate the procedure for you and make filters easier to design, but there are many reports on ASR and elsewhere that Dirac often screws up and you get strange corrections.

IMO, for the best possible quality, manual correction with linear phase FIR is the way to go. I am not disparaging MiniDSP, because if it is used well it is likely to be extremely close to a linear phase FIR and the difference may not be audible in some setups.

 

[CK.]

Thanks a lot Keith, really good information here. I will need to start playing with all these starting from a PC FIR convolver. Would you recommend equalizer APO and rephase or something else?

 

[Keith_W]

If you want linear phase FIR, these are your options from easiest to use to hardest to use:

- Dirac. The most automation but also the least flexible. Requires Dirac's own convolver since it outputs proprietary filters, not standard .WAV files. Very expensive. Lots of reports of screwy behaviour and odd corrections.
- Audiolense. Less automation than Dirac. Lets you control more aspects of correction. Lack's Acourate's flexibility.
- Acourate. This is what I use and recommend. This has a mix of manual correction and automation. Although some parts of Acourate are automated or macro based, you can do everything manually if you wish. IMO this is the most powerful DSP tool on the market, it is extremely flexible and you can do nearly anything. For e.g. Audiolense can only generate one type of crossover but with different orders. Acourate lets you generate any crossover, any order, in minimum phase or linear phase.
- REW+RePhase. I haven't explored this combo's more advanced features so I can't tell you what it can or can't do. From what I can see, it exceeds Acourate in its measurement features (e.g. it has waterfall and spectrogram, Acourate does not), but may not match it when it comes to corrections. Lots of people on ASR use this combo. This is harder to use than Acourate because it completely lacks automation, and it is fully manual so you really need to know what you are doing. It requires two different software programs and the integration is not seamless, e.g. you have to export files from one program to another and the user interfaces are very different.

There are others on the market, e.g. Focus Fidelity which is quite new and lacking some features. Until recently it did not have the ability to take measurements. It is still under development. Eclipse Audio's FIR Designer is another option, but I don't know anybody who uses it.

The four that I mentioned are the packages most commonly used on ASR. I would avoid Dirac because of its lack of flexibility, expense, and tendency to screw up. But you might prefer it because you can use it without having to learn the nitty gritty of DSP. I also do not recommend REW+RePhase for beginners for the opposite reason, you need to learn A LOT about DSP before you can use it. But it is free, and nothing beats free. Acourate and Audiolense would be my recommendations. One holds your hand more (Audiolense), the other less (Acourate).

 

[CK.]

If you want linear phase FIR, these are your options from easiest to use to hardest to use:

- Dirac. The most automation but also the least flexible. Requires Dirac's own convolver since it outputs proprietary filters, not standard .WAV files. Very expensive. Lots of reports of screwy behaviour and odd corrections.
- Audiolense. Less automation than Dirac. Lets you control more aspects of correction. Lack's Acourate's flexibility.
- Acourate. This is what I use and recommend. This has a mix of manual correction and automation. Although some parts of Acourate are automated or macro based, you can do everything manually if you wish. IMO this is the most powerful DSP tool on the market, it is extremely flexible and you can do nearly anything. For e.g. Audiolense can only generate one type of crossover but with different orders. Acourate lets you generate any crossover, any order, in minimum phase or linear phase.
- REW+RePhase. I haven't explored this combo's more advanced features so I can't tell you what it can or can't do. From what I can see, it exceeds Acourate in its measurement features (e.g. it has waterfall and spectrogram, Acourate does not), but may not match it when it comes to corrections. Lots of people on ASR use this combo. This is harder to use than Acourate because it completely lacks automation, and it is fully manual so you really need to know what you are doing. It requires two different software programs and the integration is not seamless, e.g. you have to export files from one program to another and the user interfaces are very different.

There are others on the market, e.g. Focus Fidelity which is quite new and lacking some features. Until recently it did not have the ability to take measurements. It is still under development. Eclipse Audio's FIR Designer is another option, but I don't know anybody who uses it.

The four that I mentioned are the packages most commonly used on ASR. I would avoid Dirac because of its lack of flexibility, expense, and tendency to screw up. But you might prefer it because you can use it without having to learn the nitty gritty of DSP. I also do not recommend REW+RePhase for beginners for the opposite reason, you need to learn A LOT about DSP before you can use it. But it is free, and nothing beats free. Acourate and Audiolense would be my recommendations. One holds your hand more (Audiolense), the other less (Acourate).

Thats brilliant, many thanks Keith! I would really like to know the theory behind advanced dsp and started reading dspguide.com and accurate sound reproduction using dsp by Mitch Barnett.
Recenlty finished reading Dr. Toole’s book.