I started with a ship full of max-level veterans, and I'm just a complete noob.

Director Wang was tying up the bags, while Jiang Ming was removing the seals.

That same afternoon, in Research Lab 502. A metal box sat in the center of the workbench, its lid sealed with a red seal bearing two stamps: one from the Northwest Base's Confidential Office, and the other from the Air Transport Dispatch Office.

Jiang Ming used a utility knife to cut open the seal and lift the lid. Inside, there were two layers of cotton, with a cassette tape sandwiched between them. The tape was of standard Soviet size, with a black surface and slightly curled edges.

There was a note under the tape, the handwriting was messy but legible: the last eight seconds of Operation Iron Chain, original recording from Station 3, unprocessed.

Fang Xudong stood across the worktable, his hands in the pockets of his white coat, watching Jiang Ming take out the cassette tape and then glance at the words on the note.

"What can you possibly achieve in eight seconds?"

Jiang Ming didn't respond to that. He turned around and walked to the German-made cassette recorder in the corner, put the reel on, threaded the tape, and pressed the tape transport button.

The tape recorder's head started spinning, and the sound coming from the speaker made Xiao Zhao instinctively take a step back.

That wasn't a normal recording. It sounded like someone had crammed a rusty grinding wheel and a leaky boiler into a tin shack at the same time. High-frequency shrieks and low-frequency booms were mixed together, interspersed with irregular crackling sounds, like burnt wires short-circuiting.

Eight seconds later, the conveyor belt stopped, and the speaker went silent.

Xiao Zhao rubbed his ears, and the expression on his face showed that he hadn't heard anything useful.

Fang Xudong took his hand out of his pocket, walked over, rewound the tape recorder, and played it again. This time, he put his ear close to the speaker and listened to it with his eyes closed.

"The mechanical noise from the broken generator belt, plus the suppression of broadband white noise," Fang Xudong straightened up and shook his head. "The signal-to-noise ratio is too low. If the useful signal is hidden inside, the energy is at least 20 decibels lower than the noise floor, and ordinary amplification cannot distinguish it at all."

Jiang Ming rewound the tape to the beginning and stood in front of the tape recorder without moving.

He was thinking about something.

The on-site report of Operation Iron Chain mentioned that there was a subcarrier hidden under the enemy's broadband jamming signal, with the frequency concentrated in a very narrow range, about four to six thousand hertz.

If that subcarrier does exist, it doesn't need to be very powerful; it only needs to remain stable within a specific frequency band to carry information.

The problem is how to extract it from this noise.

Ordinary LC passive filters cannot achieve this level of precision. Their quality factor is insufficient, their passband is too wide, and their edge roll-off is too slow, allowing noise to leak in from both sides and drown out the signal.

He needs an active filter to use the gain of a vacuum tube to raise the quality factor to tens or even hundreds, narrowing the passband to only a few hundred hertz, extracting signals around 5 kilohertz, and suppressing all other frequencies.

Deep in my mind, someone had mentioned a similar idea in the Tongtianlu (a historical record): a dual-T network combined with the feedback loop of a cathode follower can achieve extremely narrow bandpass characteristics in the audio band.

That's not a complete solution, just a direction. You have to calculate the remaining circuit parameters and select the components yourself.

"Little Zhao."

Xiao Zhao had just written two words in his notebook when he looked up.

"Go to the component cabinet and find mica capacitors, silver mica ones, with an error margin of less than 2%, and bring all of them with capacitance values ​​ranging from 1,000 picofarads to 0.05 microfarads, the more the better."

Xiao Zhao closed the notebook and walked out.

Fang Xudong didn't move, watching Jiang Ming take a universal test board and a roll of tinned copper wire from the tool rack.

"What are you going to do?"

"Double-T active bandpass filter." Jiang Ming placed the test board on the soldering table, then went to the cabinet and pulled out three six-pin domestic vacuum tubes, model 6J1, small voltage amplifier tubes, qualified products selected from the batch of trial production in Workshop No. 1 last year. "Three stages in series, each stage with a double-T notch network plus a cathode follower positive feedback, locking the center frequency around 5 kHz and compressing the bandwidth to within 500 Hz."

Fang Xudong walked to the welding table and bent down to look at the three 6J1s.

"How do you handle interstage coupling in a three-stage series configuration?"

"The cathode follower has a low output impedance and is directly coupled to the input of the next stage dual-T network, so there is no need for a DC blocking capacitor." Jiang Ming had already drawn the circuit diagram of the first stage on the draft paper. He drew quickly with a pencil, and the positions of the resistors and capacitors were clearly marked. "The first stage does coarse filtering with a passband of 1,000 Hz to cut off most of the mechanical noise. The second stage narrows it to 500 Hz, and the third stage reduces it to about 300 Hz, compressing it step by step."

Fang Xudong stared at the sketch for a few seconds, his lips moved, but he didn't raise any further objections.

Xiao Zhao returned carrying a cardboard box filled with small paper bags. Each bag was marked with its capacitance value and batch number in pencil. They were all silver mica capacitors, dug out from different periods of inventory.

Jiang Ming flipped through the paper bags, pushed aside all those with an error mark exceeding two percent, arranged the rest in a row according to their tolerance, and then used a multimeter to measure the actual value of each bag, picking out the ones with the smallest deviation and marking them with a red dot.

The dual-T network has extremely high requirements for capacitor matching accuracy. If the two capacitors are out of 1 percent, the notch filter frequency will shift by tens of hertz, and the center point of the entire filter will be inaccurate.

The screening took nearly forty minutes. In the end, Jiang Ming selected twelve capacitors, divided them into three groups of four, and the capacitance deviation was all within 0.5%.

Welding begins.

Jiang Ming's soldering technique is very fast. The soldering iron tip touches the solder joint for no more than two seconds. After the solder flows through, the solder joint is round and shiny, without any cold solder joints or bridging.

First, solder the three 6J1 tube sockets on, then the resistors and capacitors of the dual-T network, and finally the interstage coupling lines and power supply filters.

Xiao Zhao stood beside him, handing over the components. He checked each one on a multimeter to make sure there were no open circuits or short circuits.

Two hours later, the circuit on the test board was completed. The three-stage dual-T active filter occupied most of the board surface. The wiring was neat and there were no flying wires. A pair of shielded wires were led out from the input and output ends, and the connectors were homemade soldered plugs.

Jiang Ming connected the power supply and waited for the vacuum tube filament to light up for thirty seconds to warm up and stabilize. Then, he output a standard 5 kHz sine wave from the signal generator and connected it to the filter input. The oscilloscope probe was connected to the output.

A clean sine wave appeared on the oscilloscope screen, with an amplitude attenuation of less than one decibel.

He lowered the frequency of the signal generator to 3 kilohertz, and the amplitude of the output waveform dropped sharply, remaining less than one-tenth of its original value.

When the frequency was increased to 7 kHz, the same result was achieved: the signal was almost completely suppressed.

The passband center is 5 kHz, the negative 3 dB bandwidth is about 400 Hz, the edge roll-off is steep, and after the three-level superposition, the attenuation outside the passband exceeds 40 dB.

Fang Xudong looked at the curve on the oscilloscope, his lips twitched, but he didn't say anything.

Jiang Ming turned off the signal generator and connected the shielded cable from the monitor output of the recorder to the filter input, and the output to the oscilloscope and the recording input of another recorder.

"Play the tape."

Xiao Zhao pressed the transport button.

The oscilloscope screen initially displayed a chaotic green light, which was the waveform of all the frequency components in the eight-second recording superimposed together. It was dense with spikes, with amplitudes fluctuating wildly and showing no discernible pattern.

Then the filter starts working.

Noise outside the passband is gradually reduced, and the glitches on the oscilloscope are peeled away layer by layer as if by an invisible knife, making the waveform at the bottom clearer and clearer.

Jiang Ming rested his right hand on the fine-tuning potentiometer on the edge of the test board, while his left index finger gently tapped the table, following the residual fluctuations on the oscilloscope in a few beats.

He turned the potentiometer knob slightly by two degrees, shifting the center frequency from 5 kHz to around 4 800 Hz, and the waveform on the oscilloscope suddenly became more regular.

That's not a sine wave; it's a set of rectangular pulses with unequal intervals.

The duration varies, ranging from about 50 milliseconds to about 150 milliseconds, with intervals of varying lengths in between.

drop.

Drop drop drop.

drop.

drip.

Xiao Zhao stared at the oscilloscope screen, his mouth opening and closing repeatedly, and the pen in his hand fell and bounced on the table.

That's a Morse code waveform.

Dots and strokes form letters, letters form words, hidden beneath kilowatt-level white noise, hidden beneath the shriek of a broken generator belt, hidden beneath an eight-second recording that everyone thought was just noise.

After eight seconds, the tape recorder stopped.

Fang Xudong stood up from his chair, walked to the oscilloscope, bent down and brought his face close to the screen, as if to confirm that what he had just seen was not an illusion.

Play it again.

Jiang Ming rewound the tape and played it again. This time, he simultaneously pressed the record button on the second tape recorder, recording the filtered, clean signal separately.

After the second playback, the waveform on the oscilloscope was exactly the same as the first playback. The position and length of each dot and line matched perfectly, indicating that the signal was stable and not a false pattern formed by random noise.

Xiao Zhao squatted down and picked up the pen, his fingers clenching it tightly.

Jiang Ming turned off the filter power, took out a Morse code quick reference manual and a sharpened pencil from the drawer, and rewound the second tape recorder back to the starting point.

With the pencil tip pressed against the blank paper, he pressed the tape key, preparing to record these ticking sounds one by one as dots and strokes on the codebook.