How did the automatic tuning work in analog stereos? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: August 13, 2018, 03:05:28 am
content: So I have watched lots of videos of people fixing old stereos and one thing they never explain is how the automatic tuning works, the setting where it "locks" on to a station then will "pop" onto the next one; instead of fading in and out as you tune it. This seems like it would be really hard to do before digital.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 13, 2018, 03:07:45 am
content: It's a PLL with a narrow adjustment range.

author: BrianHG
author_rank: Super Contributor
timestamp: August 13, 2018, 03:18:50 am
content: The DC output offset of the IF stage (Raw Audio Out, before stereo separation) is VLF filtered (we want only the DC component of this signal), inverted and fed into a tuning diode on the tuner's oscillator in the RF stage. If your tuning is a little too high on the dial, the DC out of the IF is a bit high and changes the voltage on that tuning diode shifting the receive tuning a little to correct the offset. Obviously, the opposite happens when tuning is too low in frequency on the other side. This is how it use to be done in one of my old FM tuners. The selectivity of the FM IF 10.7MHz filter limits the upper and lower range. Without this, not having your tuning dead centered on the station would lead to clipping at the upper or lower limits of the IF FM decoder, or, the IF 10.7MHz ceramic filter. Changing the 'gain' on this DC signal can make that auto-tuning effect weaker, stronger, or, way too high and mess up completely.(For radios before those orange 3 pin ceramic 10.7Mhz filter.) IF chipsets which do this within themselves without adjusting the RF, in other words, shove it's internal 10.7Mhz PLL to correct there can lead to the upper and lower limits of the 10.7Mhz ceramic filter, or, this might not be a problem if you are using all tunable coil & cap filters for that 10.7Mhz band instead of the cheaper ceramic ones with their super sharp filtered frequency edges.AM is handled a little differently if it is at all implemented as it is more sophisticated. For TV tuners, which are just high frequency wideband AM radios, the TV IF has the AM out & a tuning correction out with is an FM tuner which is fed back to the tuning MCU to fine tune the TV station. Note that I know TV audio is a 4.5MHz FM carrier inside the video and is not connected to this automatic tuning correction for the main picture signal.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 13, 2018, 06:46:19 pm
content: The vintage Russian (Riga brand, I think) ones used a ratio detector (same as is used for FM demodulation) to detect a change in the IF frequency. Via a valve opamp this varied the amount of 50Hz applied to one of two windings on a shaded pole motor to physically turn the tuning knob. Clever guys, these Russians.The motor was only low power and so you could easily overcome it to retune. If you left the tuning near a station the knob would turn itself until it was spot-on.

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2018, 07:28:07 pm
content: Wow, that's a really neat but ridiculously complex way of accomplishing what it did. Could it store presets or be controlled by a remote?

author: BrianHG
author_rank: Super Contributor
timestamp: August 13, 2018, 08:06:30 pm
content: Quote from: IanMacdonald on August 13, 2018, 06:46:19 pmI once modified a stereo I had with a geared down DC motor on the tuning knob when I was 12, I used a small comparator and relay circuit on the signal strength indicator and made a seek to the previous and seek to the next radio station like in a car radios of the time.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: August 13, 2018, 10:01:34 pm
content: Took a while to track down a pic, but fairly sure it was one of these:Rigonda, not Riga. It weighed an absolute ton, needed two to lift it, and was so big that it wouldn't fit into a typical service van. So, I took the chassis back to the workshop to fix it. I forget exactly what needed doing but it was not working as was. Got it working and then fixed the auto tuning which also had a fault, think it was a leaky cap. Alignment was needed and involved some guesswork as we didn't have a manual (and it would have been in Russian anyway which none of us could read.)Surprised at the excellent sound quality. The turntable was mechanically very rough though and the playing weight on it would have quickly worn out the records. If it'd been mine I would have kept the radio but fitted a decent turntable.Owner thanked us profusely and said that the auto-tuning hadn't worked for years. Got a bottle of a good malt for my trouble.

author: Beamin
author_rank: Super Contributor
timestamp: August 18, 2018, 01:59:22 am
content: Quote from: IanMacdonald on August 13, 2018, 10:01:34 pmThe record player in that thing looks absolutely tiny. You have interesting English vernacular which part of the UK speaks like that?More reading is required before I start to understand some of these posts. VLF filter doesn't stand for very low frequency? Strange tothink the "superior TV" signal is actually just AM. What sucks about digital TV is that when it pixelates you lose the sound first making it almost unwatchable as with static you can still follow along the plot. I guess using FM sound was too much band width? I was also surprised that plugging in a fractal antenna around Washington DC I was able to pick up 35+ channels. I was expecting less then 10. Put up one of those "stellar labs $35.00" antennas (the ones where you can cut down to make the best 2 meter antenna that performs like a $300 yagi) and get a lot more. Cable would have you think over the air is dead. In LA they get 200+ channels for free. Something about watching free TV is romantic. Lets "Watch over the air broadcast and chill" as the youngsters say now a days.

author: BrianHG
author_rank: Super Contributor
timestamp: August 18, 2018, 02:32:55 am
content: Quote from: Beamin on August 18, 2018, 01:59:22 amYes, VLF is very low frequency. If you think about an FM radio signal, the broadcast modulates slower for a low voltage, higher for a higher voltage coming out of the FM IF section/demodulator. This signal is also known as the Base-band signal. If your tuning is off one way, the output of the IF demodulator voltage goes higher. It goes lower as your tuning on the station is too low. Actually, the same would happen if your tuning is centered, but, the FM broadcast happen to be sending say something like a 1 Hz tone as the output would swing up and down in voltage very slowly. The output of the FM demodulator is usually put through a cap in series to remove this DC low frequency offset in case your tuning isn't dead center. However, if you filter out all the high frequencies (ie the audio in the broadcast) on this output, and only keep the DC, you now have a DC voltage offset based on your tuning position offset (Note: if the FM broadcast has a 0.1hz tone in it, this cap would also probably go up and down in voltage once every ten seconds). This DC is all too easy to extract, just take the output of the IF demodulator, place it through a 100K resistor and then a 10uf cap to GND. The voltage on that cap will have a DC voltage position based on your tuning position. To remove the DC and get just the sound, place 1 uf cap in series at the output of the demodulator and tie a 100k resistor to GND on the other side. At the output of that cap, there is no DC, just the higher frequency, or the audio.As for TV, yes, it's a 6MHz bandwidth AM broadcast as for when you have reflections in you receiver antenna, since video is so much faster than audio, down to 70ns, since the reflections have traveled a different distance from the broadcast tower to your TV antenna, ghosts will appear, but, the picture is still visible. If it were an FM broadcast, not only will there need to be more parts since TV started in the vacuum tube era, but you will get speckles all over your image instead of a ghost transmition as the FM receiver tries to discriminate between what to show in the mix of 2 or more signals of the same image coming in at different times. Things get worse if you have 2 overlaying broadcasts mixed together. For TV and over the air, AM is a only viable solution. The embedded 4.5Mhz audio needs to be narrow and low quality since it is so close to that color chroma at 3.57Mhz. It is also easier in AM transmitter and receiver to get the close to the full broadcast bandwidth of each channel.As for analog satellite TV, since the broadcast was line of sight with no reflections, so they could broadcast in FM. This is why, with a C/KU band analog dish with a cheap proper analog receiver, the broadcast was near lossless, as if you had a wired connection to the source. The audio also was now moved to 6.2 Mhz and 6.8 Mhz, independent stereo with full 200Khz FM bandwidth on each channel making for near CD quality audio if your receiver's 2 audio FM tuners were good enough. Sometime there would be 4 channel audio with additional sound channels at 5.2Mhz, 5.8 Mhz, and even more in the 7 and 8 Mhz as the bandwidth for analog satellite TV was usually up to 25Mhz per channel, though, usually the first 5 to 6 Mhz was reserved for the picture allowing close to 600 lines of resolution with good equipment. The scrambled channels on analog satellite TV f--ked everything up as they messed with the black levels and color burst level. Those idiots at General Instruments who made the VCRS scrambling system developed the video level system for some weird Japanese non-standard Betamax video output levels and messed with the colors since they regenerated the color reference color burst signal at broadcast which was needed to be used as a clock to sync for their encrypted digital audio and the video output had the video sync and color burst regenerated on the decoder card in your receiver which never matched the original broadcast source levels messing up the tint and saturation and with the regenerated sync amplitudes, they messed up the black level.

author: tooki
author_rank: Super Contributor
timestamp: August 19, 2018, 10:36:11 am
content: Quote from: IanMacdonald on August 13, 2018, 06:46:19 pmNeato!I have an early-90s Sony mini stereo system (individual components, but connected via proprietary ribbon cables, so highly integrated) that has an alarm function including preset volume. And it does that by actually turning on a few seconds before the alarm set time, using the volume pot motor to first return the volume to zero, then actually turning on the playback source, and then turning up the volume pot to the setpoint.

author: Beamin
author_rank: Super Contributor
timestamp: August 19, 2018, 11:19:37 am
content: Quote from: tooki on August 19, 2018, 10:36:11 amThat's how you know you have a good receiver/amp the remote control turns the volume knob.Speaking of which I have a marantz amp I paid a lot of money for back in 1999. I think it was made in Korea or some non 3rd world country. Would that need recapping after sitting for 5 or 6 years? It's not old enough to be vintage but old enough not to be new. That's something you repair not replace. I think it was having issues with one set of channels but I didn't really get to see if it was connectors. It has an A/B switch but it's A OR B you can't power four speakers like the cheap amps. Took me a while to figure out if A and B are both set to on neither will work. What was this thread about?

author: SeanB
author_rank: Super Contributor
timestamp: August 19, 2018, 05:19:26 pm
content: Being 1999 that puts it in the middle of Cap plague territory. Would suggest a look inside and look at spot values for smaller caps, and that the bigger ones are not bulging or leaking. If they are out of tolerance or leaking then replace all of the same value and voltage at least, or all of them. However, being linear supplies in the amplifier at least, and with no major SMPS either in most cases, you can use regular electrolytics, not low ESR types as replacements.

author: Beamin
author_rank: Super Contributor
timestamp: August 19, 2018, 07:34:40 pm
content: Quote from: SeanB on August 19, 2018, 05:19:26 pmHave to put in an order for the" nitchycon" ones MrCarlson loves so much. For my ears there are no cleaner sounding amps.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 20, 2018, 02:45:01 am
content: Quote from: Beamin on August 18, 2018, 01:59:22 amThank the decision to use oddball 8-VSB in the US for that. Elsewhere, OFDM is used which is considerably more robust to narrowband interference. Implement OFDM with a skewed power distribution and it will even degrade gradually with signal fading. Another thing is that since real world video content rarely uses all the bandwidth in the channel, they could have implemented best effort parity packets to add robustness against short bursts of noise.The way analog video degrades gradually is a big reason it's still used for FPV on model aircraft. With interest in HD FPV, some have worked on sending video as raw packets over Wifi. Think of it as basically UDP but even lower level, as in the Wifi link itself is "connectionless".

author: IanB
author_rank: Super Contributor
timestamp: August 20, 2018, 02:50:36 am
content: Quote from: Beamin on August 18, 2018, 01:59:22 amTo what are you referring? I don't see anything out of the ordinary.

author: IanB
author_rank: Super Contributor
timestamp: August 20, 2018, 03:00:15 am
content: Quote from: Beamin on August 18, 2018, 01:59:22 amIn analog days, British TV used to broadcast high quality stereo sound as part of the TV signal ("NICAM stereo"), but this has now been superseded by digital broadcasting.At times in the UK they would also broadcast sound on FM radio simultaneously with the TV picture ("simulcast") when showing concerts and musical events where good sound quality was desirable. This would allow you to play the sound over your hi-fi system without having to connect up the TV as an external source.

author: BrianHG
author_rank: Super Contributor
timestamp: August 25, 2018, 04:42:18 am
content: Quote from: IanB on August 20, 2018, 03:00:15 amThis was done her in Canada, or at least Montreal as well. It was used for concerts and Grammys, or something similar of the time back in the late 70s/early 80s. If you hooked up your cable to your FM receiver, there was also not only FM radio, but, a few TV channels in stereo. This stopped once we went beyond 63 channels because the FM Band was being used for some extra channels numbered at 95 through 99...See Midband, channels after 6, comes 95 through 99.https://en.wikipedia.org/wiki/North_American_television_frequencies