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

"The seventh one, 4.81 MHz, stayed for six seconds, and then jumped away."

Lu Qinghe pressed his thumb against the tuning knob, and his index finger followed the dial little by little. The knurled copper edge of his fingertip rubbed red marks on his fingertip, and all that could be heard in the receiver was the howling that swept across the entire frequency band.

Jiang Ming wrote 4.81 on the seventh line, and casually jotted down the short burst of noise before the switch. The pencil tip was dried by the cold wind, leaving intermittent graphite marks on the paper.

"The eighth one, 5.56 MHz, eight seconds."

A thick line lit up at the edge of the frequency band, then swept back towards the center. The enemy station did not advance in a fixed direction; each jump was like someone casually turning a turntable.

Fang Xudong drew eighty channels as a disk, pinned the first eight frequency points to the perimeter in sequence, and then connected them with straight lines to obtain an entangled broken line graph.

"If it were to recursively follow the circumference, it should rewind from the lower limit after crossing the upper limit, but point seven breaks this condition."

He wiped off the disc and replaced it with the dual-mode shift model from the Soviet military textbook, separating the odd-numbered positions from the even-numbered positions and calculating two sets of step lengths separately.

The error for odd-numbered positions reaches 260 kHz in the fourth term, and the error for even-numbered positions also exceeds the receiver bandwidth, making it impossible to extrapolate either sequence further.

"The ninth one, 5.17 MHz, eleven seconds."

Lu Qinghe tightened the earphones again, and her right hand no longer followed the scale completely. She would first listen to the offset of the feedback center, and then use the knob to make up the remaining distance.

The wind outside the carriage lifted the hem of his coat, and the thumbtacks began to loosen. Jiang Ming pressed down the graph paper with the heel of his palm and wrote the frequency and dwell time of the ninth group into the table.

Fang Xudong turned to the second half of the textbook and applied the prime number wheel model commonly used in gear scramblers, treating 10 kilohertz as a tooth position, and tried to restore the cycle using a 19-tooth and 23-tooth gear set.

The first six items barely fell within the error range, but the seventh item crossed two teeth that should have been continuously engaged, and the eighth item jumped back in the opposite direction, causing the model to lose its constraints on the spot.

He pushed the slide rule back to its original position and replaced it with the third type of mechanical cam cipher model. However, the length of the dwell time and the frequency did not have a fixed correspondence, and the cam profile could not be pushed out.

"The tenth one, 5.44 MHz, pause for four seconds."

After Lu Qinghe finished reporting the data, he removed his hand from the knob and wiped the sweat from the edge of the copper plate with his sleeve. A thin, bright red skin had already been worn down to his fingertips.

Ten numbers covered the graph paper, and the linear step, prime number wheel and double sequence recursion were all crossed out, leaving only intersecting black lines on the paper.

Fang Xudong closed the Soviet military textbook, the cover hitting the iron frame with a dull thud.

"At this point, we can rule out mathematical codebooks; it must be using hardware noise."

Old Sun, who was standing guard by the power supply switchboard, turned around when he heard this.

"Hardware also needs a way of functioning; can a machine roll dice by itself?"

"Resistor thermal noise, thyristor discharge noise, and contact jitter of vibrating contacts can all be used as inputs for the next tuning."

Fang Xudong tapped ten sets of data with a pencil, but found no correlation between the frequency span and the dwell time, and even the positive and negative values ​​of adjacent differences did not have a stable period.

"If they let the noise voltage charge the capacitor, and then the comparator determines which position the relay should be in, the result each time will be determined by the physical state at that time."

"If the machine itself can't calculate the next jump, we certainly can't."

Lu Qinghe scanned the frequency band again. The interference band was spreading out to both sides, and the base radio station was swallowed up by noise as soon as it sent out half a call sign.

"Then we can only wait for it to finish jumping before chasing it again, but the receiver needs time to switch frequencies, so we can only hear a fragment of a sentence at most each time."

Fang Xudong picked up the walkie-talkie, his thumb already on the transmit button.

"I informed Director Wang that the password could not be predicted, and requested that the encircling troops be withdrawn to the mouth of the ditch, and that we switch to wired communication after dawn."

"Don't report it yet."

Jiang Ming removed the graph paper from the box, walked into the direction finding vehicle, and his gaze fell on the three-stage dual-T filter next to the receiver.

This filter was originally responsible for suppressing broadband noise floor and only retaining the main bright bands formed when the enemy station hops its frequency. In the past, direction finding relied entirely on it to clean the signal.

"Lu Qinghe, cut off the filter bypass."

"After cutting it open, all the background noise will come in, and the headphones can't handle it."

"Reduce the gain first, then switch to the bypass; I want to hear the moment the frequency changes."

Lu Qinghe glanced at him, turned the volume knob down to the lowest setting, and then flipped open the copper jumper next to the dual-T filter.

Unwashed, raw noise flooded the carriage, the loudspeakers rapidly rising from a low, electrical hum, causing the metal walls to vibrate subtly.

She gradually increased the gain, and the piercing whistling, mixed with diesel engine noise, feeder induction noise, and enemy station sweep noise, buried any effective signal underneath.

Old Sun covered his left ear and stood outside the car door to guard the power supply cabinet, while Fang Xudong also stepped back half a step.

"What can you hear from inside?"

"Don't speak."

Jiang Ming sat down in front of the receiver, placed the ten frequency points on his lap, closed his eyes, and tried to discern the edge changes left by each frequency change in the noise.

When the enemy station moved away from 5.44 MHz, two soft sounds came from the loudspeaker, very close to each other. The first sound was sharp and the second sound was slightly lower. Then the broadband noise spread to the new frequency.

That's not the signal content, but rather the transient noise that leaks into the transmitter through the control circuit when the tuning capacitor finishes charging and the relay changes its contacts.

Before the next frequency hopping, a soft sound appears again, but the interval is slightly longer than the previous group, and the tail of the second sound is dragged out in the opposite direction.

Jiang Ming picked up a pencil and drew two small dots next to the dwell time in the tenth line.

"For the eleventh one, you don't need to report the full frequency; just report the interval between the two noises before the switch."

Lu Qinghe widened the oscilloscope's time reference, allowing the frequency hopping edge to fill the screen, so that the light spot formed two vertical lines not far apart when it passed by.

"One hundred and eighty milliseconds."

Jiang Ming wrote down the numbers, and then retrieved the edges that had been erased by the filter during the first ten switching cycles segment by segment from the paper tape record.

Although the main signal on the paper tape has been cleaned, slight notches still remain at the moment of frequency switching, and the spacing between the notches is weakly related to the direction of frequency jump.

Fang Xudong leaned down and looked at it for a while, then slowly lowered the walkie-talkie in his hand.

"If it is indeed thermal noise input, the relay's operating time will not be related to the next frequency point."

"So it's not truly random."

Jiang Ming converted all ten frequency points into offsets relative to 4.8 MHz, and then set the upward jump to positive and the downward jump to negative.

Inside the mine, the technical supervisor sat at the control panel, watching the frequency hopping curves generated by the new codebook spread across the recording paper.

The main codebook relies on the golden ratio stepping, while the standby machine uses two sets of asynchronous capacitor networks. The frequency wheel will reverse every time it encounters a boundary, so the outside can only obtain discrete results without any rewinding traces.

After checking the output power, the operator locked the spare local oscillator constant temperature bath at the rated scale.

"The other side is still scanning the frequency; they've already lost their eleventh time."

"Let them sweep."

The technical supervisor picked up the enamel mug; the coffee grounds inside were not fully dissolved, and brown particles clung to the rim of the mug.

"This kind of sequence has only been used twice in Europe, and no one in the Far East has ever seen it. They don't even know where the randomness comes from."

Inside the command post, the communications officer removed one earpiece and repeatedly called the First Battalion of Beigou, but only received a barrage of noise in response.

"Beigou has completely lost contact. The last report was at the second fork in the road, and no call sign has been received since."

The other radio station also could not receive the signal from the Second Battalion of Xigou. The two red flags on the map were located six kilometers apart, with weathered rock hills guarded by the mine in between.

Director Wang pressed down on the edge of the table, the veins on the back of his hand bulging, but he didn't pick up the walkie-talkie to urge Tu Qiu on his way out.

The mechanical clock on the wall was still ticking. Each time the gears fell, the second hand moved forward one notch, and only thirty-four minutes remained until the forty-minute time limit.

"Keep calling, but lower the transmission power. Don't let the enemy determine the company's location through the echo."

The communications officer put his headset back on and started calling in turn using short call signs. No one spoke next to the map.

Inside the direction-finding vehicle, Jiang Ming wrote down the differences between adjacent frequency points in the first column, then subtracted the adjacent differences from each other to list the second layer of changes.

The differences in the first layer are still chaotic. In the second layer, the absolute values ​​of the first and eighth terms are close, the second and seventh terms are in opposite directions, and the third and sixth terms fall within the same ratio zone.

He folded the ten points back from the middle, making the beginning and end overlap on the paper one by one. The differences that were originally inexplicable began to reveal the remnants on both sides of the mirror that were opposite to each other.

The pencil stopped at the fifth item, and the crease on the paper passed right through the two sets of offsets.

Deep within his consciousness, the scroll of the successful candidate in the imperial examinations opened on its own, and a line of dark gold characters appeared on the new, unlit page.

[Genius premonition activated: Please pay attention to the symmetrical afterimages.]