Thank you Sir!

Glad I could help!

Mike, looking at the signal directly off the playback head is possible, but technically difficult. The reason it is difficult is that the bias signal is much higher and stronger than the actual audio signal. So what your would see on an oscilloscope would be the bias signal (typically 100kHz) being slightly modulated by the audio signal. The example I showed in the video is just that, a example that is exaggerated for illustrative purposes. However, I recently procured a tunable filter that I believe will be able to block the high frequency bias signal completely and leave just the audio signal. Which, would among other things make head alignment more precise. So, if I am successful in doing that I will post a "part 2" to my Tape Bias video.

@@yellowspringsvintageaudio352 Agreed. I'd like to see all the signal coming off the head, even the bias signal.

Looking at the graph at 4:54 into the video. If the bias is too low, the signal is shifted to the left. This means that part of the music will be on the non-linear portion of the tape - which leads to distortion of the sound that is not recorded in the linear portion. If the bias is too high, the sound will be recorded on the portion of the tape that is to the right of the linear portion - this means some of the sound will be at the saturation level of the tape and therefore will be compress and have no dynamic range.

@@yellowspringsvintageaudio352 thank you!

@@CyrilleBoucanogh I used the book "The Science of Sound Recording" by Jay Kadis, as a reference. The book is available as a paperback on Amazon and other online sources. Alternatively, you can find much of the information in the book from this Stanford University web site: ccrma.stanford.edu/~jay/subpages/NewFoundations.html

@@yellowspringsvintageaudio352 I'm very grateful for your help. I've ordered the book and going to check the site too. Thanks a lot!

Miro, if you look at the signal coming directly off the playback head, what you see is the stronger, higher frequency, bias signal being amplitude modulated by the audio signal. The first circuit the playback signal goes through is a filter that blocks the higher bias frequency... leaving the audio signal to proceed on to amplifier and metering circuits. The opposite is true during recording. The last circuit before the record head is one that amplitude modulates the much stronger and higher bias frequency with the audio signal. So the answer to your question is, YES. That is exactly what happens. Thanks for your question and watching my video.

Hi Steven. I pulled the service manual up on your cassette deck and examined the bias adjustment and the CrO/normal circuitry for both the bias and the equalization. The way Hitachi works, is pretty much like most cassette decks do. The bias adjust, which there are separate ones for the Right channel and the Left channel, is made using CrO (Type II) tape. When the tape selection is placed in the "normal" position, a different resistor is placed in the bias circuitry that lowers the bias voltage. Based on your service manual, when the CrO, type II and the Hitachi is recording, you adjust RT4L and RT4R so that there is 15 volts of bias signal across the left and right record/playback head, respectively. For the "normal" record there is no separate adjustment, just slightly different circuit that lowers the bias voltage. What you are experienceing may be an Equalization problem. There are no adjustments on your deck for equalization. The only difference in equalization when switching from "normal" to CrO are different sets of fixed value "resistor/capacitor tank" circuits. Another common reason you might experience "dull" sound could be a need for a good head cleaning and demagnetization. If you haven't tried that, I would do it before preceding to make any "adjustments". Finally, if you don't have the service manual for your deck, you can download it for free from www.hifiengine.com. You will have to register to download, but the registration is just used to prevent someone from downloading more than 5 documents a day. There is a sister site that covers turntables... it is www.vinylengine.com. Steven, I hope this helps you! Please don't hesitate to contact me at: ysvintageaudio@gmail.com

​@@yellowspringsvintageaudio352 Hi, thanks so much for this amazingly helpful reply! The fixed circuit differences for type I and II bias and eq make sense to me. I do have the service manual, and after cleaning, demagnetizing, and aligning the heads, I followed all the calibration steps. The only one I did differently was the bias step. I wasn't sure where to measure the voltage across both ends of the head, so instead I recorded and played back 1khz and 10khz tones at -20dB using a type II tape that I wanted to calibrate the deck for, and adjusted the L and R bias pots so that there was no difference in the levels of the low and high frequency tones on playback. I would be interested in the 15v method if I understood where to make the measurement though. One thing I've noticed is that if I record a type II tape on this deck with type II bias and eq selected, it plays back correctly on other decks --- but if I play a type II tape in this deck that was recorded in any of my other decks, I have to select type I EQ on this deck or the highs are absent. This issue is not present with type I tapes. Both pre-recorded and home-recorded type I tapes play back perfectly on this deck with type I bias and type I EQ selected. So I'm thinking it might indeed be an EQ problem. Thanks again!

@@rumsfeldisinsane - Keep in mind EQ is only related to Bias if the bias adjustment is such that it limits the changes to the frequency response curve made by the EQ circuits. For example, if the EQ is trying to increase the high frequencies, but in doing so the signal reaches tape saturation (because the bias is too high) the high frequencies will be compressed and cut off. The same would be true for low frequencies if the bias was too low. As I said in the video - the object is to keep the recorded signal in the linear response range of the tape. That is what the bias adjustment does. Type II tapes have a greater linear portion, so by increasing the bias you are provided the capability to take fuller advantage of the potential dynamic range of the tape. The Type II EQ settings is fulfillment of that advantage via extra dynamic range - but if the bias is not adjusted correctly then the EQ efforts may not be successful. NOTE: When I say dynamic range, I am not only talking about distortion-free frequency response, but also about being able to capture the very slightest of sounds along with the very loudest of sounds.

Fine explanation. I thoroughly enjoyed this .

@JD-lk7im Thank you! I am glad you enjoyed it. If you have other vintage Audio topics of interest, please let me know.