APRS = Advance Precise Recording-level System
ATC = Automatic Tape Calibration
Basically what happens is that when you insert a blank cassette tape into the recorder and press the ATC button the recorder will automatically record a number of test sequences onto the blank tape, then read the test sequences back and calculate from the results what the optimal bias and equalisation settings are, it will then record new test signals onto the tape, then read them off the tape to verify that the original calculations were correct, and if they are not redo the whole process until an optimal solution is found. When optimal results have been archived they are then stored in memory and are used while that tape is in use. Some high end recorder models even store information on the beginning of the tape so when you reinsert the tape later on for rerecording or playback they will optimally adjust to the tape.
This is highly useful for a number of reasons, tapes are designed to meet certain characteristics set forward in standards dictated by Philips, but in the real world can differ quite a lot for various reason. Some tapes were developed to enable you to use chrome or similar formulations on older or cheaper recorders that do not support anything but recording onto Normal tapes, these tapes have notches like a Normal tape and will function fine under the either Normal or Chrome settings but optimally are somewhere in-between. Between 1975 and 1985 there were a number of formulations available that required either high or low bias and most recorders up until ca. 1982 would allow you to switch the bias but almost no modern recorders have that option, most manufacturers altered their tape formulations to work better with standardised bias settings but some did not, even recently made Fuji-Magnetics tapes sound best with very high biasing for instance. Some manufacturers like BASF made tape formulations that were designed to be flexible with their bias requirements.
But there are more issues there, tapes can vary from a batch to batch, some tape manufactures were using manufacturing techniques from the stone age, the Ampex factory was using knifes to distribute the magnetic layer onto a tape which meant that tapes could wary within a batch even. The cemical compositon of the magnetic layer may vary between batches as well especially with smaller manufacturers and cheaper product lines. More advanced manufacturers like BASF and TDK had very tight quality controls in place so that did not happen but in those cases the cemical composition of tape was often being continuslly improved without any changes to product names or packaging, BASF in particular had a tendency to do this. In addition some tape suppliers did not manufacture any tape, most just re-branded ready made cassettes from other manufacturers, some bought pancakes intended for tape duplication and loaded them into shells and some bought bulk rolls and cut them and loaded into shells. In those cases you could be buying a Mifa or Boots branded cassette that had the same packaging as last year but then contained BASF tape but this year had one from Philips and the year after one from ORWO.
In other words not only do tapes vary from manufacturer to manufacturer, they can also have different characteristics even if they come from the same maker.
Most recorders either use the notches in the cassette tape shell to decide what sort of tape you have inserted into the machine and will switch between pre-set bias and Eq curves, others will allow or require you to switch between the tape types you have inserted, usually giving you a choise between Normal (ferric), Chrome and Metal types and will apply pre-set bias and Eq curves in response to this. A few recorders will allow you to switch between Low, Normal and High bias settings in addition to having Normal, Chrome and Metal Eq setting and a few recorders have a variable bias control but none have any manual Eq compensation for recording (Playtrim is for playback only). Each recorder was aligned at the factory to optimally function with a certain brand and type of tape, this is usually either a tape from the manufacturer of the device, their business partners or a market leading brand and type in their home market. So you have a specific brand and type of tapes that will optimally work with your recorder, but the problem is that the formulation for that tape may have changed since your recorder was made, it may not be sold where you live or have gone from the market altogether.
This means that depending on what recorder you have, you may have no or at best only very rudimentary control over the Eq characteristics and ranging from no to reasonably good control over biasing. This becomes a problem when you have no access to optimal formulations, while it is not a disaster, your recorder will record onto just about anything after all, the quality is often sub-par. Furthermore even when you have fully variable bias adjustment knob it can be a pain to adjust on a 2 head recorder since you will have to repeatedly record onto a tape, wind back to evaluate the results and then redo the whole thing again for verification. Easier on a 3 head machine since you can monitor the results while you are adjusting.
Automatic Tape Calibration changes the game completely since it allows your recorder to use any tape you can find while getting close to optimal results, this is especially welcome on cheaper recorders even though their ATC circuits are often much more rudimentary than those on more expensive recorders and in some cases even have limited amount of flexibility when it comes to Eq settings. But even so cheap cassette recorders are exactly where you would see just about any junk used to record on and comparing recorders side by side that have the same frame and transport but with the more expensive version having ATC of some sort, Sony TC-WE475 and Sony TC-WE475 come to mind, the difference can be quite dramatic with more problematic tapes.
Make sure you do not use the ATC button on tapes you have already recorded on since it will record over the beginning of the tape, remember to break off the recording tabs on the cassette after you have recorded onto it.
...and of course the term can be used to mean automatic calibration of open reel tape recorders which is a slightly more complex routine but very few models actually accomplished this.
Super Auto BLE
What differs with Pioneer's BLE routine is that the recorder also tests for optimal recording levels, but in most other recorders the recording level is left up to you and in high end recorders you sometimes get a Limiter circuit that ensures that the tape is not overloaded, but has no effects on levels otherwise. The Pioneer decks that feature this function have a computer controlled potentiometer for level control of the audio signal rather than routing the audio signal through the record level potentiometer as is the usual practice, or in other words the potentiometer hooked up to the record level knob controls another potentiometer rather than the strength of a signal directly. This enables the recorder to take control of the signal strength that is sent to the tape and while you control the overall levels what happens if you have the BLE circuitry scan the tape before recording is that it ensures that recording levels and are matched to the tape characteristics and it will reduce a signal that would otherwise overload the tape, it will also ensure that the metering is correct and that bias and Eq correct in relation to signal strength, but optimal bias and Eq may be dependent on recording levels.
Note that this is done by using level control only, not by using compression, so it does not have a detrimental effects on the audio signal. Make sure you do not use the BLE on tapes you have already recorded on it will record over the beginning of the tape, remember to break off the recording tabs off the tops of the cassettes you have already recorded onto.
An improved version of the process was introduced in ca 1994 and was called Super Auto BLE XD that does a further check on the tape after it has done the BLE routine, checking to see if high level signals improve with variable biasing, if they do variable biasing is introduced, but that improves primarily the dynamic range in signals above the mid-range and in this case the XD stood for eXtended Dynamic range. In all cases where this system was implemented you could defeat the XD extension and have the recorder just carry out BLE.
Care and feeding of your cassette deck
A nice article from Churchsoundcheck.com.
Getting the grunge off your tape heads