 In the past I have heard several room correction systems during demos and they never impressed me. They replaced acoustical problems with DSP problems back in the day. So I lost interest. Over the past year I came across some interesting demos and tried some things myself. As Dylan said, the times they are changing. This video has been recorded on Friday, July 27th, 2018, the day after the hottest night ever measured in my country, 23.4 degrees Celsius, 74.1 degrees Fahrenheit. Since we have a mild climate we are not equipped for these temperatures. That is why this video is late and why you might hear a fan blowing in the back. The temperature here, my little studio in the attic, is about 45 degrees Celsius, 113 degrees Fahrenheit. So far for the public service messaging. Let's go to the topic at hand. Room correction in general tries to compensate for acoustic problems by applying filtering to the source material. This can only work in the low frequencies, roughly where the length of the sound waves is half the distance between the boundaries of a room, or multiples of that. Depending on the religion you adhere to, this works for frequencies up to 200 to 500 Hz. It's not coincidence that I mention those frequencies in my loudspeaker placement videos. It's good practice to watch one of them and put them into practice before even considering room correction. Over loudspeaker placement can prevent room problems and thus need not be corrected. Or even might not be correctable by filtering. Although proper placement will not prevent all problems, it most likely will prevent the big problems leaving only smaller problems to be corrected by electronic means. All room correction systems basically do the same. They measure the so called transfer function of the audio channel from the DA converter to the measurement mic at the listening position. This means that the non-linearities in the chain, DAC, amplifier, speakers and acoustics will be corrected, or at least an attempt will be made. Given the fact that corrections are only made below 500 Hz or lower, there will be little corrections of electronics, but speakers might be corrected to a degree. It also depends on where the correction takes place. If it takes place in a computer that plays back the music, it really will be the entire chain, DAC, amplifier, speakers and acoustics. But if the room correction takes place in the amplifier, for instance, it will only potentially influence the power amp, the speakers and the acoustics. There are at least two ways to do the correction. In the amplitude domain or in the time and amplitude domain. The first system uses parametric equalizers. These equalizers are called this way since all parameters of the filter can be varied. Frequency, boost or cut and Q-factor. The first two will be clear. The third one describes the relation between the center frequency and the bandwidth over which the attenuation takes place. A high Q stands for a small band filter while a low Q stands for a wide filter. And there are variations over this theme where the filter width stays constant when varying the cut or boost and filters that vary the width depending on the cut or boost. There are even filters that use one variant when boosting and the other while cutting. These are uniquely to the concert sound world. Since all filtering for room correction nowadays is done digitally, it is relatively easy to add all kinds of filtering, appropriate for a given situation, providing sufficient DSP horse power is present and the manufacturer has designed or bought sufficient DSP programs to do so. Measuring the amplitude versus frequency, when done properly, gives you a good impression of what deviation in the frequency domain are to be corrected. But with some more calculations it can also become clear what time alignment would improve the sound. Of course filters might cause face distortion and group delays. In layman's terms the highs, mids and lows are delayed differently, potentially destroying transient response and stereo imaging. The better the filters are chosen, the less this will occur. But it is also possible, although more complex, to include a time domain in the filtering, keeping the energy constant and again when done properly, resulting in a yet more transparent sound image. Since modern room correction is applied digitally, there are software music players like Amara, HQ Player, J-River, Fubar, Roon and even Volumio on the Raspberry Pi that can take care of it, sometimes needing a so called plug-in. If you already have the software, adding the filtering is very affordable as we will see in the coming videos. If you don't want to use a computer or use non-computer playback equipment like CD player streamers and the like, you could use a box in between the player and the amplifier. That room correction box might be analog or digital in and out, but in all cases up to here they only work with the audio source selected. An alternative might be to use a room correction device or computer in between the tape out and taping of your amp while listening with the tape monitor switch engaged at all time. Putting the device between the preamp and the power amp is also done but would not be my first choice since chances are they have to work with very low levels and thus have limited resolution. Let's see again where room correction can be applied. This is the schematics of a reproduction chain, the sum of the stereo and room acoustics. If you use a computer as a source, software in the computer can do the room correction filtering. If your player's software does not support this or if you use another digital source like a CD player, you could use a room correction device that has digital in and out in between the digital source and the DAC. In both these cases the room correction will only work with the digital source and for instance not with the TV that is connected to the same stereo. That is not the case if you use an amplifier or AV receiver that has room correction built in. If not you can connect a room correction device over analog to the tape monitor connectors of the amp and have the tape monitor engaged all the time. The disadvantage here is that the room correction device has to do the analog to digital conversion prior to filtering and convert it back to analog after filtering. Room correction is no matter for me and a completely different application of knowledge I partly have but also partly lack. I am already working on four different systems to see what they can and can't do. As soon as I feel comfortable to report on them you will see the videos appear. In the meantime I might publish other videos depending on what review equipment becomes available. So, if you want to follow my quest, subscribe to this channel or follow me on Twitter, Facebook or Google+. If you liked this video, please consider supporting your channel through Patreon or PayPal. Every financial support is much appreciated. The links are in the comments. Help me to help even more people enjoy music at home by telling your friends on the web about this channel. I am Hans Beekhuyzen, thank you for watching and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music.