Inconsistent PTS Gap in AXIS P1375 RTSP Stream

[vivekatoffice]

Using an AXIS P1375 Network Camera for vehicle speed detection. The camera is configured with 1080p resolution, H.264 codec, 25 FPS, and 6 Mbps bitrate. Zipstream is disabled, and the firmware version is 11.11.124.

While grabbing the stream via the RTSP protocol, I expect a consistent presentation timestamp (PTS) gap of 40 ms between frames, as per the 25 FPS configuration. However, we have observed inconsistent gaps—sometimes as high as 100 ms and as low as 2 ms—which result in inaccurate speed calculations.

Please let me know if you have some suggestions to resolve this issue or if it is a limitation.

I have tested the latest AXIS Q1728 Block Camera (AXIS OS version 12.5.56) with the following code:

import cv2
import time

# Replace with your RTSP URL
rtsp_url = "rtsp://root:pass@10.176.12.99/axis-media/media.amp"

cap = cv2.VideoCapture(rtsp_url)

if not cap.isOpened():
    print("Error: Cannot open stream.")
    exit()

prev_pts = None
frame_count = 0

print("Starting frame timestamp logging... Press Ctrl+C to stop.")
try:
    while True:
        ret, frame = cap.read()
        if not ret:
            print("Failed to grab frame.")
            break

        # Get timestamp in milliseconds
        pts = cap.get(cv2.CAP_PROP_POS_MSEC)

        if prev_pts is not None:
            gap = pts - prev_pts
            print(f"Frame {frame_count}: PTS = {pts:.2f} ms, Gap = {gap:.2f} ms")
        else:
            print(f"Frame {frame_count}: PTS = {pts:.2f} ms")

        prev_pts = pts
        frame_count += 1

        # Optional: delay if needed
        # time.sleep(0.01)
except KeyboardInterrupt:
    print("\nStopped by user.")

cap.release()

Requirements:

pip install opencv-python

Output:

Starting frame timestamp logging... Press Ctrl+C to stop.
Frame 0: PTS = 0.00 ms
Frame 1: PTS = -166.67 ms, Gap = -166.67 ms
Frame 2: PTS = -133.33 ms, Gap = 33.33 ms
Frame 3: PTS = -100.00 ms, Gap = 33.33 ms
Frame 4: PTS = -66.67 ms, Gap = 33.33 ms
Frame 5: PTS = -33.33 ms, Gap = 33.33 ms
Frame 6: PTS = 0.00 ms, Gap = 33.33 ms
Frame 7: PTS = 33.33 ms, Gap = 33.33 ms
Frame 8: PTS = 66.67 ms, Gap = 33.33 ms
Frame 9: PTS = 100.00 ms, Gap = 33.33 ms
Frame 10: PTS = 133.33 ms, Gap = 33.33 ms
Frame 11: PTS = 166.67 ms, Gap = 33.33 ms
Frame 12: PTS = 200.00 ms, Gap = 33.33 ms
Frame 13: PTS = 233.33 ms, Gap = 33.33 ms
Frame 14: PTS = 266.67 ms, Gap = 33.33 ms
Frame 15: PTS = 300.00 ms, Gap = 33.33 ms
Frame 16: PTS = 333.33 ms, Gap = 33.33 ms
Frame 17: PTS = 366.67 ms, Gap = 33.33 ms
Frame 18: PTS = 400.00 ms, Gap = 33.33 ms
Frame 19: PTS = 433.33 ms, Gap = 33.33 ms
Frame 20: PTS = 466.67 ms, Gap = 33.33 ms
Frame 21: PTS = 500.00 ms, Gap = 33.33 ms
Frame 22: PTS = 533.33 ms, Gap = 33.33 ms
Frame 23: PTS = 566.67 ms, Gap = 33.33 ms
Frame 24: PTS = 600.00 ms, Gap = 33.33 ms
Frame 25: PTS = 633.33 ms, Gap = 33.33 ms

NTP settings in camera:

[github-actions[bot]]

This automatically generated reply acts as a friendly reminder.

Answers to your questions will most often come from the community, from developers like yourself. You will, from time to time, find that Axis employees answers some of the questions, but this is not a guarantee. Think of the discussion forum as a complement to other support channels, not a replacement to any of them.

If your question remains unanswered for a period of time, please revisit it to see whether it can be improved by following the guidelines listed in Axis support guidelines.

[rocky-ji]

Two consideration:

1. Is the camera running with high CPU? So try to minimun the CPU, request less streaming, limit the bandwidth, limit/stop SD card recording. maybe change the compression from 30 to 40.
2. Is your network situation good? Try to change a switches to see the result

[Cacsjep]

Cool 😃😃

[vivekatoffice]

python SEI_time.py > Logs.txt

import av
from urllib.parse import quote
import datetime

username = "root"
password = quote("pass")
ip = "10.176.12.17"

rtsp_url = f"rtsp://{username}:{password}@{ip}/axis-media/media.amp?videocodec=h264&onvifreplayext=1"

options = {
    "rtsp_transport": "tcp",
    "max_delay": "500000"
}
# AXIS P1375 Network Camera AXIS OS version 11.11.160
container = av.open(rtsp_url, options=options)

print("Looking for timestamp gaps...")

prev_pts = None
frame_count = 0
expected_gap_sec = 1 / 30  # For 30 FPS
tolerance_sec = 0.001      # Allow ±1 ms variation

for packet in container.demux(video=0):
    for frame in packet.decode():
        pts = frame.pts
        time_base = frame.time_base

        if pts is None or time_base is None:
            print(f"Frame {frame_count}: No timestamp info.")
            frame_count += 1
            continue

        # Convert PTS to float seconds
        timestamp_sec = float(pts * time_base)

        if prev_pts is not None:
            prev_timestamp_sec = float(prev_pts * time_base)
            gap = timestamp_sec - prev_timestamp_sec
            is_consistent = abs(gap - expected_gap_sec) <= tolerance_sec
            status = "--> Consistent" if is_consistent else "--> Inconsistent"
            print(f"Frame {frame_count}: Delta_t = {gap:.6f} sec  {status}")
        else:
            print(f"Frame {frame_count}: Timestamp = {timestamp_sec:.6f} sec")

        prev_pts = pts
        frame_count += 1

Attaching the logs from today's test:
AXIS_P1375_Network_Camera_AXIS_OS_version_11_11_160_logs_377686_frames.zip

[vivekatoffice]

I have tested the PTS with VLT camera AXIS P1375 : AXIS Virtual Loan Tool

Logs attached for the reference.
VLT_AXIS_P1375_Network_Camera_AXIS_OS_version_11_11_160_logs_31July_2025.zip

[vivekatoffice]

Assume you have pcapdump.cap in the same folder which is captured using following options:

Capture the Wireshark trace on the client side. i.e., the PC where the client software is running. This is to confirm that the actual video data has been received by the client.

import pyshark
import base64

cap = pyshark.FileCapture('pcapdump.cap', display_filter='rtp')

previous_timestamp = None
seen_timestamps = set()

for pkt in cap:
    try:
        timestamp = int(pkt.rtp.timestamp)

        # Only process new RTP timestamps
        if timestamp in seen_timestamps:
            continue
        seen_timestamps.add(timestamp)

        # Get RTP payload as base64 string and decode it
        payload_b64 = pkt.rtp.payload.replace(':', '')  # Remove colons
        payload_bytes = bytes.fromhex(payload_b64)

        # Extract NAL unit type from first byte
        nal_header = payload_bytes[0]
        nal_type = nal_header & 0x1F

        print(f"Time={timestamp} | NAL={nal_type}", end='')

        if previous_timestamp is not None:
            diff = timestamp - previous_timestamp
            print(f" | Diff={diff}")
        else:
            print()

        previous_timestamp = timestamp

    except (AttributeError, IndexError, ValueError):
        continue

Logs file for reference:

difflogs.txt

[vivekatoffice]

Trying to check if FFmpeg can be used for displaying the PTS diff:

Make sure to keep the ffmpeg.exe in the same folder.

import subprocess
import re

ffmpeg_path = r"E:\Vivek\Demo\PTS\ffmpeg.exe"
rtsp_url = "rtsp://root:pass@10.176.12.17/axis-media/media.amp?videocodec=h264&onvifreplayext=1&fps=25"

timebase = 90000  # Common for H264
fps_filter = "fps=fps=25"

# ---- REGEX ----
nal_re = re.compile(r"nal_unit_type:\s*(\d+)\(([^)]+)\)")
pts_re = re.compile(r"in pts (\d+)")

# ---- STATE ----
first_rtp_timestamp = None
previous_pts = None
frame_count = 0
nal_type = None

print("Reading frames with PTS and NAL unit types...\n")

cmd = [
    ffmpeg_path,
    "-rtsp_transport", "tcp",
    "-i", rtsp_url,
    "-vf", fps_filter,
    "-f", "null", "-",
    "-loglevel", "debug"
]

proc = subprocess.Popen(cmd, stderr=subprocess.PIPE, stdout=subprocess.DEVNULL, bufsize=1, text=True)

try:
    for line in proc.stderr:
        line = line.strip()
        if not line:
            continue

        # NAL type extraction
        nal_match = nal_re.search(line)
        if nal_match:
            nal_id = int(nal_match.group(1))
            if nal_id == 5:
                nal_type = "I"
            elif nal_id == 1:
                nal_type = "P"
            else:
                nal_type = f"?{nal_id}"

        # PTS extraction
        pts_match = pts_re.search(line)
        if pts_match:
            pts = int(pts_match.group(1))
            frame_count += 1

            if first_rtp_timestamp is None:
                # Use this PTS as first RTP ts
                first_rtp_timestamp = pts

            raw_rtp = pts + first_rtp_timestamp  # reverse engineered "raw" RTP timestamp
            rtp_delta = pts  # since: raw_rtp - first_rtp = pts

            time_sec = pts / timebase
            diff_str = ""

            if previous_pts is not None:
                diff = pts - previous_pts
                diff_str = f" | Diff={diff}"

            print(f"Frame {frame_count}: Type={nal_type} | PTS={pts}{diff_str} | Time={time_sec:.3f}s")

            previous_pts = pts

except KeyboardInterrupt:
    print("\nStopped by user.")

finally:
    proc.terminate()
    proc.wait()

Output (using AXIS P1375 Network Camera: AXIS OS version 11.11.160):

Frame 31348: Type=P | PTS=112845851 | Diff=3599 | Time=1253.843s
Frame 31349: Type=P | PTS=112849452 | Diff=3601 | Time=1253.883s
Frame 31350: Type=P | PTS=112853051 | Diff=3599 | Time=1253.923s
Frame 31351: Type=P | PTS=112856653 | Diff=3602 | Time=1253.963s
Frame 31352: Type=P | PTS=112860251 | Diff=3598 | Time=1254.003s
Frame 31353: Type=P | PTS=112863853 | Diff=3602 | Time=1254.043s
Frame 31354: Type=P | PTS=112867452 | Diff=3599 | Time=1254.083s
Frame 31355: Type=P | PTS=112871053 | Diff=3601 | Time=1254.123s
Frame 31356: Type=P | PTS=112874652 | Diff=3599 | Time=1254.163s
Frame 31357: Type=P | PTS=112878253 | Diff=3601 | Time=1254.203s
Frame 31358: Type=P | PTS=112881852 | Diff=3599 | Time=1254.243s
Frame 31359: Type=P | PTS=112885451 | Diff=3599 | Time=1254.283s
Frame 31360: Type=P | PTS=112889052 | Diff=3601 | Time=1254.323s
Frame 31361: Type=P | PTS=112892651 | Diff=3599 | Time=1254.363s
Frame 31362: Type=P | PTS=112896253 | Diff=3602 | Time=1254.403s
Frame 31363: Type=P | PTS=112899852 | Diff=3599 | Time=1254.443s
Frame 31364: Type=P | PTS=112903453 | Diff=3601 | Time=1254.483s
Frame 31365: Type=P | PTS=112907052 | Diff=3599 | Time=1254.523s
Frame 31366: Type=I | PTS=112910653 | Diff=3601 | Time=1254.563s
Frame 31367: Type=P | PTS=112914252 | Diff=3599 | Time=1254.603s
Frame 31368: Type=P | PTS=112917851 | Diff=3599 | Time=1254.643s
Frame 31369: Type=P | PTS=112921452 | Diff=3601 | Time=1254.683s
Frame 31370: Type=P | PTS=112925051 | Diff=3599 | Time=1254.723s

PTS_difference_dump.txt

[vivekatoffice]

Feel free to re-open it OR share the solution/alternative/comment if already resolved👁️‍🗨️